Energy Conversion and Management - UMEXPERT 178 · PDF fileReview A review on pyrolysis of...

Energy Conversion and Management 115 (2016) 308–326

Contents lists available at ScienceDirect

Energy Conversion and Management

journal homepage: www.elsevier .com/locate /enconman

Review

A review on pyrolysis of plastic wastes

http://dx.doi.org/10.1016/j.enconman.2016.02.0370196-8904/� 2016 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +60 162709281; fax: +60 379675319.E-mail addresses: [email protected] (S.D. Anuar Sharuddin), [email protected] (F. Abnisa), [email protected] (W.M.A. Wan Daud), mk_aroua@um

(M.K. Aroua).

Shafferina Dayana Anuar Sharuddin, Faisal Abnisa ⇑, Wan Mohd Ashri Wan Daud,Mohamed Kheireddine ArouaDepartment of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e i n f o a b s t r a c t

Article history:Received 14 December 2015Accepted 12 February 2016

Keywords:Plastic wastesPyrolysisLiquid productFuelEnergy recovery

The global plastic production increased over years due to the vast applications of plastics in many sectors.The continuous demand of plastics caused the plastic wastes accumulation in the landfill consumed a lotof spaces that contributed to the environmental problem. The rising in plastics demand led to the deple-tion of petroleum as part of non-renewable fossil fuel since plastics were the petroleum-based material.Some alternatives that have been developed to manage plastic wastes were recycling and energy recov-ery method. However, there were some drawbacks of the recycling method as it required high labor costfor the separation process and caused water contamination that reduced the process sustainability. Dueto these drawbacks, the researchers have diverted their attentions to the energy recovery method to com-pensate the high energy demand. Through extensive research and technology development, the plasticwaste conversion to energy was developed. As petroleum was the main source of plastic manufacturing,the recovery of plastic to liquid oil through pyrolysis process had a great potential since the oil producedhad high calorific value comparable with the commercial fuel. This paper reviewed the pyrolysis processfor each type of plastics and the main process parameters that influenced the final end product such asoil, gaseous and char. The key parameters that were reviewed in this paper included temperatures, typeof reactors, residence time, pressure, catalysts, type of fluidizing gas and its flow rate. In addition, severalviewpoints to optimize the liquid oil production for each plastic were also discussed in this paper.

� 2016 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3092. Pyrolysis of plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309

2.1. Polyethylene terephthalate (PET) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3102.2. High-density polyethylene (HDPE). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3102.3. Polyvinyl chloride (PVC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3112.4. Low-density polyethylene (LDPE). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3112.5. Polypropylene (PP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3112.6. Polystyrene (PS). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3122.7. Mixed plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312

3. Process parameters condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312
3.1. Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3133.2. Type of reactors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313
3.2.1. Batch and semi-batch reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3133.2.2. Fixed and fluidized bed reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3143.2.3. Conical spouted bed reactor (CSBR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3153.2.4. Microwave-assisted technology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316

3.3. Pressure and residence time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3173.4. Catalysts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317

.edu.my

http://crossmark.crossref.org/dialog/?doi=10.1016/j.enconman.2016.02.037&domain=pdf

http://dx.doi.org/10.1016/j.enconman.2016.02.037

mailto:[email protected]




http://dx.doi.org/10.1016/j.enconman.2016.02.037

http://www.sciencedirect.com/science/journal/01968904

http://www.elsevier.com/locate/enconman

S.D. Anuar Sharuddin et al. / Energy Conversion and Management 115 (2016) 308–326 309

3.4.1. Catalyst importance in pyrolysis of plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3173.4.2. Type of catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317

3.5. Type and rate of fluidizing gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320
4. Characteristics of plastic pyrolysis oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320
4.1. Physical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3204.2. Chemical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321

5. By-products of the plastic pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322
5.1. Char . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3225.2. Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323
6. Discussion on plastic pyrolysis scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3237. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324

Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324

1. Introduction

Plastic plays a vital role in enhancing the standard lives ofhuman being for more than 50 years. It is a key of innovation ofmany products in various sectors such as construction, healthcare,electronic, automotive, packaging and others. The demand of com-modity plastics has been increased due to the rapid growth of theworld population. The global production of plastic has reachedabout 299 million tons in 2013 and has increased by 4% over2012 [1]. The continuous rising of plastic demand led to the grow-ing in waste accumulation every year. It was reported that 33 mil-lion tons of plastic waste are generated in the US based on 2013statistic [2]. As in Europe, 25 million tons of plastic ended up inwaste stream during the year of 2012 [1]. Based on the statisticestablished in Europe, about 38% of the plastic waste still wentto the landfill, 26% were recycled while 36% were utilized forenergy recovery [1]. This shows that the percentage of plasticwaste ended up in the landfill still very high that it occupied a hugespace. Plastics may take up to billions of years to degrade naturally.They degrade gradually since the molecular bonds containinghydrogen, carbon and few other elements such as nitrogen, chlo-rine and others that make plastic very durable. The continuous dis-posal of plastic in the landfill would definitely cause seriousenvironmental problem.

In order to reduce plastic disposal to the landfill, recyclingmethod is considered as another alternative to manage plasticwaste. Back to the statistic mentioned above, the percentage ofrecycling still at the lowest. Recycling plastic has proven difficultand it can be costly because of the constraints on water contamina-tion and inadequate separation prior to recycle that is labor inten-sive [3]. Separation is needed since plastics are made of differentresin compound, transparency and color. Normally, pigmented ordyed plastics have lower market value. Clearly transparent plasticsare often desirable by the manufacturers since they can be dyed totransform into new products, thus have greater flexibility [4]. Withthe stringent requirement to get high value product, recycling plas-tic becomes quite challenging nowadays.

Although plastic recycling able to reduce some amount of plas-tic waste, the more reliable and sustainable method has beenestablished. Since high demand of plastics have been received eachyear, the reduction of fossil fuel such as coal, gas and especiallypetroleum that made up plastic itself has gained great interest ofmany researchers to discover and develop potential energyresources due to the rising in energy demand. Some of the newenergy resources that have been explored include solar energy,wind power, geothermal and hydropower technology. Recently,the energy conversion from waste has been an intelligent way tofully utilize the waste to meet the increased energy demand. Theconversion of plastics to valuable energy is possible as they arederived from petrochemical source, essentially having high calori-

fic value. Hence, pyrolysis is one of the routes to waste minimiza-tion that has been gaining interest recently.

Pyrolysis is the process of thermally degrading long chain poly-mer molecules into smaller, less complex molecules through heatand pressure. The process requires intense heat with shorter dura-tion and in absence of oxygen. The three major products that areproduced during pyrolysis are oil, gas and char which are valuablefor industries especially production and refineries. Pyrolysis waschosen by many researchers since the process able to produce highamount of liquid oil up to 80 wt% at moderate temperature around500 �C [5]. In addition, pyrolysis is also very flexible since the pro-cess parameters can be manipulated to optimize the product yieldbased on preferences. The liquid oil produced can be used in mul-tiple applications such as furnaces, boilers, turbines and dieselengines without the needs of upgrading or treatment [6]. Unlikerecycling, pyrolysis does not cause water contamination and isconsidered as green technology when even the pyrolysis by-product which is gaseous has substantial calorific value that itcan be reused to compensate the overall energy requirement ofthe pyrolysis plant [7]. The process handling is also much easierand flexible than the common recycling method since it does notneed an intense sorting process, thus less labor intensive.

Many research papers have been published regarding thepotential of various types of plastics in pyrolysis processes for liq-uid production. It should be noted that the product yield and qual-ity heavily depends on the set up parameters. Therefore, thisreview focused on different type of plastic pyrolysis that has beenexplored together with the main affecting parameters in plasticpyrolysis process that need an attention in order to maximize liq-uid oil production and enhance the oil quality. The main parame-ters include temperature, type of reactors, residence time,pressure, different catalysts usage and type of fluidizing gas withits flow rate. Additionally, some relevant discussion regarding theoptimization of liquid oil yield was also presented in this paper.

2. Pyrolysis of plastics

Fundamentally, different types of plastics have different com-positions that normally reported in terms of their proximate anal-ysis. Proximate analysis can be defined as a technique to measurethe chemical properties of the plastic compound based on four par-ticular elements which are moisture content, fixed carbon, volatilematter and ash content [8]. Volatile matter and ash content are themajor factors that influence the liquid oil yield in pyrolysis process.High volatile matter favored the liquid oil production while highash content decreased the amount of liquid oil, consequentlyincreased the gaseous yield and char formation [7]. Table 1 sum-marized the proximate analysis of different plastics. Based onTable 1, it was observed that the volatile matter for all plastics is

Table 1Proximate analysis of plastics [7].

Type of plastics Plastics type marks Moisture (wt%) Fixed carbon (wt%) Volatile (wt%) Ash (wt%) Ref.

Polyethylene terephthalate (PET) 0.46 7.77 91.75 0.02 [9]0.61 13.17 86.83 0.00 [10]

High-density polyethylene 0.00 0.01 99.81 0.18 [11]0.00 0.03 98.57 1.40 [10]

Polyvinyl chloride (PVC) 0.80 6.30 93.70 0.00 [12]0.74 5.19 94.82 0.00 [10]

Low-density polyethylene 0.30 0.00 99.70 0.00 [13]– – 99.60 0.40 [14]

Polypropylene 0.15 1.22 95.08 3.55 [15]0.18 0.16 97.85 1.99 [10]

Polystyrene 0.25 0.12 99.63 0.00 [16]0.30 0.20 99.50 0.00 [13]

Polyethylene (PE) 0.10 0.04 98.87 0.99 [15]Acrylonitrile butadiene styrene (ABS) 0.00 1.12 97.88 1.01 [17]Polyamide (PA) or Nylons 0.00 0.69 99.78 0.00 [17]Polybutylene terephthalate (PBT) 0.16 2.88 97.12 0.00 [10]

310 S.D. Anuar Sharuddin et al. / Energy Conversion and Management 115 (2016) 308–326

very high while the ash content is considered low. These character-istics indicate that plastics have high potential to produce largeamount of liquid oil through pyrolysis process. Since the resultsof plastics proximate analysis are very convincing, the followingdiscussion would focus more on the process parameters involvedduring the pyrolysis process that would have major influence inthe liquid production.

2.1. Polyethylene terephthalate (PET)

PET has become the great choice for plastic packaging for vari-ous food products, mainly beverages such as mineral water, softdrink bottle and fruit juice containers. This is due to its intrinsicproperties that are very suitable for large-capacity, lightweightand pressure-resistant containers. Other applications of PETinclude electrical insulation, printing sheets, magnetic tapes, X-ray and other photographic film [18]. The extensive applicationsof PET would cause an accumulation of PET waste in the landfill.Recycling PET waste was the current practice of handling accumu-lated plastic waste. However, the bulkiness of the containerscauses high frequency of collections and therefore, increases thetransport costs. To ease the recycling process, the PET waste needsto be sorted into different grades and colors that make its recoveryinefficient and uneconomical. Hence, other alternative for PETrecovery such as pyrolysis process has been explored and the pro-duct yield was analyzed by several researchers.

Cepeliogullar and Putun [19] have explored the potential of PETin pyrolysis process to produce liquid oil using fixed-bed reactor at500 �C. The heating rate was 10 �C/min and nitrogen gas was usedas the sweeping gas in this experiment. It was observed that theliquid oil yield was lesser than the gaseous product. The liquidoil obtained was 23.1 wt% while the gaseous product was 76.9 wt% with no solid residue left. The low liquid yield could be explainedthrough the proximate analysis based on Table 1, showing the rel-atively low volatile content of PET around 86.83% in comparisonwith other plastics in which the volatile contents were all above

90%. Unfortunately, almost half of the oil composition containedbenzoic acid which was around 49.93% based on the gas chro-matography–mass spectroscopy (GC–MS) analysis. The acidic char-acteristic in pyrolysis oil was unfavorable due to its corrosivenessthat deteriorated the fuel quality [19]. Besides that, benzoic acidwas a general sublime that could clog piping and heat exchanger,thus need a serious attention if running in industrial scale [20,21].

On the other hand, Fakhrhoseini and Dastanian [5] foundslightly higher liquid oil yield at the same operating temperatureand heating rate. The liquid yield obtained was 39.89 wt%, gaseouswas 52.13 wt% and solid residue was 8.98 wt%. Therefore, it can beconcluded that the liquid oil production from the PET pyrolysisobtained in the ranges of 23–40 wt% while gaseous yield in theranges of 52–77 wt%. Based on these results, PET might be the mostsuitable plastic to be used in pyrolysis if gaseous product became apreference, for instance to provide energy supply to heat up thereactor at the desired temperature.

2.2. High-density polyethylene (HDPE)

HDPE is characterized as a long linear polymer chain with highdegree of crystallinity and low branching which leads to highstrength properties. Due to its high strength properties, HDPE iswidely used in manufacturing of milk bottles, detergent bottles,oil containers, toys and more. The various applications contributeabout 17.6% in plastic waste category which is the third largestplastic type found in municipal solid waste (MSW) [22]. HDPEwastes have a great potential to be used in pyrolysis process sinceit can produce high liquid yield depends on the set up parameters.Many studies have been conducted on HDPE pyrolysis at differentoperating parameters to investigate the product yield obtained.

Ahmad et al. [23] explored the pyrolysis study of HDPE usingmicro steel reactor. The pyrolysis temperatures were within 300–400 �C at heating rate of 5–10 �C/min. Nitrogen gas was used asthe fluidizing medium. From the experiment, they found that thehighest total conversion happened to be at 350 �C with liquid


was the dominant product yield (80.88 wt%). The solid residue wasvery high at 300 �C (33.05 wt%) but the amount was reducing to0.54 wt% at the highest temperature of 400 �C.

On the other hand, Kumar and Singh [24] have done the thermalpyrolysis study of HDPE using semi-batch reactor at higher tem-perature range of 400–550 �C. It was observed that the highest liq-uid yield (79.08 wt%) and gaseous product (24.75 wt%) obtained attemperature of 550 �C while wax started to dominate in productfraction at higher temperature of 500–550 �C. The dark brownishoil obtained from the pyrolysis had no visible residue and the boil-ing point was from 82 to 352 �C. This suggested the mixture of dif-ferent oil component such as gasoline, kerosene and diesel in theoil that matched the properties of conventional fuel as shown inTable 2. Besides, the sulfur content in the HDPE pyrolytic oil wasvery low (0.019%) that made it cleaner to the environment.

Besides that, Marcilla et al. [26] have also studied the HDPEpyrolysis at 550 �C using batch reactor. The liquid oil yield was84.7 wt% and gaseous product around 16.3 wt%. This results pro-ven that higher liquid oil could be obtained at higher temperaturebut there was also a limitation that should be noted. Too high tem-perature would reduce the liquid oil yield and increased the gas-eous product since the process had passed the maximumthermal degradation point. Mastral et al. [27] conducted the HDPEpyrolysis in a fluidized bed reactor at 650 �C and they found thatthe liquid oil production was around 68.5 wt% and 31.5 wt% gas-eous product. This shows that the liquid was cracked to gaseouswhen further heated up at a very high temperature above 550 �C.

2.3. Polyvinyl chloride (PVC)

Unlike other thermoplastics such as polyethylene (PE), poly-styrene (PS) and polypropylene (PP) which can be softened byheating and solely derived from oil, PVC is exceptional since it ismanufactured from the mixture of 57% chlorine (derived fromindustrial grade salt) and 43% carbon (derived from hydrocarbonfeedstock such as ethylene from oil or natural gas) [28]. The chlo-rine property makes PVC an excellent fire resistance, thus verysuitable for electrical insulation. The compatibility PVC to be mixedwith many additives makes it a versatile plastic. Regular applica-tions of PVC include wire and cable insulation, window frames,boots, food foil, medical devices, blood bags, automotive interiors,packaging, credit cards, synthetic leather, etc. Even though it haswide applications, the research done on the PVC pyrolysis foundin the literature was very less due to the dangerous substance thatit tend to release when heated at high temperature.

Miranda et al. [29] conducted the pyrolysis of PVC in a batchreactor at temperature range of 225–520 �C and heating rate of10 �C/min. The experiment was done under vacuum and total pres-sure of 2 kPa was applied. Liquid oil obtained was not that high andvaried from 0.45 wt% to 12.79 wt% as the temperature increased.Tar accumulation was even higher than the liquid oil obtainedand the amount kept increasing up to 19.6 wt%. Hydrogen chloride(HCl) was found to be the main product obtained from the exper-iment with the highest yield of 58.2 wt%. HCl tend to be corrosive

Table 2Comparison of HDPE pyrolytic oil and conventional fuel properties.

Type of oil HDPE pyrolytic oil properties[24]

Conventional fuel properties[25]

Boiling point (�C) Cv (MJ/kg) Boiling point (�C) Cv (MJ/kg)

Gasoline 82–352 42.9 40–200 43.4–46.5Kerosene 150–300 43.0–46.2Diesel 150–390 42.8–45.8

and toxic when heated moderately that caused damage to the pro-cess equipment. This was one of the main reasons that led to theshutdown of the pyrolysis pilot plant in Ebenhausen, Germany[29].

Therefore, it can be concluded that PVC was not preferable forpyrolysis since the yield of liquid oil was very minimum. Further-more, PVC waste accumulated in MSW was very minimal, aboutless than 3% in plastic waste category which was very limited[22]. Additionally, the release of harmful product such as HCl andthe presence of chlorinated compound such as chlorobenzene inthe pyrolysis liquid could be toxic to the environment. To over-come this, a dechlorination process of PVC was required to reducethe chlorine content in liquid oil. This process could be achievedthrough several methods such as stepwise pyrolysis, catalyticpyrolysis and pyrolysis with adsorbents added to the PVC sample[30]. Hence, the pyrolysis of PVC required an additional cost whenan extra dechlorination step was needed which was one of the dis-advantages to the industry.

2.4. Low-density polyethylene (LDPE)

In contrast to HDPE, LDPE has more branching that results inweaker intermolecular force, thus lower tensile strength and hard-ness. However, LDPE has better ductility than HDPE since the sidebranching causes the structure to be less crystalline and easy to bemolded. It has an excellent resistance to water, thus widely appliedas plastic bags, wrapping foils for packaging, trash bags and muchmore. All these items are commonly used in our daily lives andtherefore, LDPE waste has been accumulated day by day that it isknown as the second largest plastic waste in MSW after PP [22].As one way to recover energy and reduce waste, pyrolysis of LDPEto oil product has received much attention by researchersnowadays.

Bagri and Williams [31] have investigated the LDPE pyrolysis infixed-bed reactor at 500 �C with heating rate of 10 �C/min. Theexperiment was done for duration of 20 min and nitrogen was usedas fluidizing gas. It was observed that high liquid yield of 95 wt%was obtained with low gas yield and negligible char. High liquidoil yield of 93.1 wt% has also been obtained by Marcilla et al.[26] when the experiment was carried out in a batch reactor at550 �C, but this time with lower heating rate of 5 �C/min.

There are also some researchers who studied the LDPE pyrolysisat lower operating temperature less than 500 �C. From the researchconducted by Uddin et al. [32] using batch reactor at 430 �C, theliquid yield obtained was around 75.6 wt%. Aguado et al. [33] haveobtained a closer yield with Uddin et al. [32] which was 74.7 wt%when using the same type of reactor at 450 �C. However, the liquidoil yield could be increased when pressure was applied in the reac-tor during the process, even though at lower temperature. This wasproven by Onwudili et al. [34] who used pressurized batch reactor(0.8–4.3 MPa) in LDPE pyrolysis at 425 �C. From the experiment,they have obtained 89.5% liquid oil, 10 wt% gaseous and 0.5 wt%char. This indicates that pressure may have an influence on thecomposition of pyrolysis product that would be discussed after-ward in this paper.

2.5. Polypropylene (PP)

PP is a saturated polymer with linear hydrocarbon chain thathas a good chemical and heat resistance. Unlike HDPE, PP doesnot melt at temperature below than 160 �C. It has a lower densitythan HDPE but has higher hardness and rigidity that makes itpreferable in plastic industry. PP contributes about 24.3% in plasticwastes category which are the largest amount of plastics found in


MSW [22]. The diverse applications include flowerpot, office fold-ers, car bumpers, pails, carpets, furniture, storage boxes and more.The high demand of PP in daily life causes the amount of PP wastesto increase each year and therefore, pyrolysis of PP is one of themethods that can be used for energy recovery. Several researchershave investigated the pyrolysis of PP at various parameters to mea-sure the liquid oil yield and properties.

In a study conducted by Ahmad et al. [23] on PP pyrolysiswithin 250–400 �C using micro steel reactor, they summarized thatthe highest liquid oil was achieved at temperature of 300 �Caround 69.82 wt% with total conversion of 98.66%. The increasein temperature to 400 �C only reduced the total product conversionto 94.3% and increased solid residue from 1.34 to 5.7 wt%. Thisindicates that coke formation started to dominate at higher tem-perature. However, Sakata et al. [35] have explored the PP pyroly-sis at higher temperature of 380 �C. They found higher liquid yieldof 80.1 wt%, 6.6 wt% gaseous and 13.3 wt% solid residue left. On theother hand, Fakhrhoseini and Dastanian [5] obtained higher liquidyield about 82.12 wt% when performed PP pyrolysis at 500 �C. Nev-ertheless, further increase in temperature more than 500 �C onlyreduced the liquid yield collected. This was proven by Demirbas[36] who carried out the PP pyrolysis at extreme temperature of740 �C in a batch reactor which resulted in 48.8 wt% liquid yield,49.6 wt% gaseous and 1.6 wt% char.

2.6. Polystyrene (PS)

PS is made of styrene monomers obtained from the liquid petro-chemical. The structure consists of a long hydrocarbon chain withphenyl group attached to every other carbon atom. PS is naturallycolorless but it can be colored by colorants. It is heat resilience andit offers reasonable durability, strength and lightness that makethis polymer desirable to be used in variety of sectors such as infood packaging, electronics, construction, medical, appliances andtoys. The wide range of applications signifies the large wasteamount of PS in MSW accumulated each year. Unfortunately, PSis not included in the roadside recycling program in which therecycling bins only included glasses, papers, cans, and certain plas-tics. Even though there is a plastic category, normally people willnot throw the foam food packaging into plastics recycle bin andthey often go to the general bin. Thus, PS is generally not separatedand not economically to collect for recycling due to its low densitypolystyrene foam. Hence, the only way the PS waste can be fullyutilized is through pyrolysis process in which it can be turned intomore valuable oil product rather than to end up in the landfillsforever.

Onwudili et al. [34] have investigated the pyrolysis of PS in abatch pressurized autoclave reactor at 300–500 �C for one hourduration. The heating rate used was 10 �C/min and the experimen-tal pressure given was 0.31 MPa up to 1.6 MPa. From the experi-ment, they found that the PS pyrolysis produced a very highliquid oil yield around 97.0 wt% at optimum temperature of425 �C. The maximum amount of gas produced was only 2.5 wt%.

The high yield of liquid oil product was also supported by Liuet al. [37]. The difference was during this time, the pyrolysis ofPS was conducted using fluidized bed reactor at temperature of450–700 �C. The highest liquid oil obtained was 98.7 wt% at600 �C. Nevertheless, the amount of liquid oil produced was alsoconsidered high at lower temperature of 450 �C which was around97.6 wt% and it differed by only 1.1 wt%. In the case when energysaving was the priority, lower temperature was preferable as itcould reduce the energy cost incurred. Based on the study doneby Demirbas [36], the liquid oil reduced to 89.5 wt% when the PSpyrolysis was running at 581 �C in a batch reactor. Therefore, thePS pyrolysis was not recommended to run at temperature morethan 500 �C to optimize the liquid oil production.

2.7. Mixed plastics

As previously mentioned, pyrolysis process has an addedadvantage over the recycling process since it does not need anintense sorting process. In recycling process, most plastics arenot compatible with each other to be processed together duringrecycling. For instance, a slight amount of PVC contaminant pre-sent in PET recycle stream will degrade the whole PET resin bybecoming yellowish and brittle that requires reprocessing [38].This shows that recycling process is very sensitive to contaminantsthat it requires all plastics to be sorted based on type of resins, col-ors and transparency. However, pyrolysis process seems to bemore sustainable since liquid oil still can be produced from themixed plastics in the feedstock. This has been encountered by sev-eral researchers who conducted studies of mixed plastics pyrolysis.

Kaminsky et al. [39] studied the pyrolysis of mixed plasticwastes collected from German households which was composedapproximately 75% of polyolefins (PE, PP) and 25% PS. There wasindeed a small amount of PVC content remained in the materialafter the separation step about less than 1 wt% and this was shownby the presence of the chlorine content in the product yield. Theexperiment was conducted in a fluidized bed reactor at 730 �Cwhich finally produced 48.4 wt% liquid oil. The amount of liquidoil obtained was very similar to the study conducted by Demirbas[36] in pyrolysis of polyolefins (PP, PE) and PS mixture collectedfrom landfill which was approximately 46.6 wt%. The gaseousand solid yields were reported to be 35 wt% and 2.2 wt% respec-tively. In terms of the oil composition, it contained 4 ppm chlorineresulted from the remaining PVC left in the material. However, itdid not deteriorate the oil quality since the minimum chlorinelimit in petrochemical processing was less than 10 ppm. Further-more, the rest of the chlorine content was found to be the largestin solid residue. Therefore, the author concluded that the chlorinecontent in the feedstock could not be more than 1 wt% to ensurehigh quality oil was produced.

The potential of polyolefins mixed plastics in pyrolysis wasalso explored by Donaj et al. [40]. The mixed plastics were com-posed of 75 wt% LDPE, 30 wt% HDPE and 24 wt% PP. The experi-ment was operated at high temperatures of 650 �C and 730 �Cin a bubbling fluidized bed reactor. The results showed that theliquid obtained was higher at lower temperature of 650 �C whichwas around 48 wt%. However, this oil fraction consisted of 52%heavy fraction such as heavy oil, wax and carbon black. In con-trast, it was up to 70% light fraction of liquid contained in thepyrolysis oil (44 wt%) running at 730 �C. This means that thehigher the temperature, the lighter the hydrocarbon liquid orgaseous produced. Therefore, it should be noted that there wasa tremendous change in the product distribution when the tem-perature was further increased.

In comparison to the single plastic pyrolysis, it can be seen thatthe pyrolysis of mixed plastics produced lower liquid yield lessthan 50 wt%. Nevertheless, the quality of oil produced was compa-rable to the single plastic pyrolysis in terms of the oil compositionthat made it ideally suited for further processing in petrochemicalrefineries.

3. Process parameters condition

Parameters play major role in optimizing the product yield andcomposition in any processes. In plastic pyrolysis, the key processparameters may influence the production of final end productssuch as liquid oil, gaseous and char. Those important parametersmay be summarized as temperature, type of reactors, pressure, res-idence time, catalysts, type of fluidizing gas and its rate. Thedesired product can be achieved by controlling the parameters at


different settings. In-depth discussions of the operating parame-ters are reviewed in the following subsections.

3.1. Temperature

Temperature is one of the most significant operating parame-ters in pyrolysis since it controls the cracking reaction of the poly-mer chain. Molecules are attracted together by Van der Waals forceand this prevents the molecules from collapsed. When tempera-ture in the system increases, the vibration of molecules insidethe system will be greater and molecules tend to evaporate awayfrom the surface of the object. This happens when the energyinduced by Van der Waals force along the polymer chains is greaterthan the enthalpy of the C–C bond in the chain, resulted in the bro-ken of carbon chain [41].

The thermal degradation behavior of the plastics can be mea-sured using thermogravimetry analyzer. The analyzer producestwo types of graphs which are thermogravimetry analysis (TG)curve and derivative thermogravimetry analysis (DTG) curve. TheTG curve measures the weight change of substance as function oftemperature and time [24]. On the other hand, the DTG curve givesthe information on the degradation step occurred during the pro-cess which is indicated by the number of peaks [24]. In the PETpyrolysis study conducted by Cepeliogullar and Putun [19], theyobserved that the main PET degradation started at 400 �C with avery small weight change occurred when the temperature was inthe range of 200–400 �C. The maximum weight loss of the sub-stance happened at temperature of 427.7 �C. No significantchanges were observed at temperature more than 470 �C. There-fore, the authors concluded that the thermal degradation of PEThappened at temperature range of 350–520 �C.

As for the HDPE, Chin et al. [42] reported the thermal degrada-tion started at 378–404 �C and was almost completed at 517–539 �C based on the thermogravimetry analysis (TG) at differentheating rates in the range of 10–50 �C/min. Higher heating ratesspeeds up the weight loss, thus increases the rate of reaction. Inanother thermal behavior study carried out by Marcilla et al.[43], they found that the maximum degradation rate of HDPEoccurred at 467 �C. This important temperature needs to put intoconsideration when running the pyrolysis experiment to ensurethe most optimum liquid yield.

On the other hand, PVC may have different thermal behaviorthan others when the major weight loss occurred at two differenttemperature ranges as reported by Cepeliogullar and Putun [19].The first temperature range was between 260 and 385 �C in whichmaximum weight loss of 62.25% from the initial weight occurred.The second temperature range happened to be between 385 and520 �C where about 21.74% weight loss from the original weight.As the temperature was raised up till 800 �C, the weight loss ofthe substance became insignificant (1.62%). Thus, it was concludedthat the degradation temperature of PVC was in the range of 220–520 �C [19].

In LDPE pyrolysis, Marcilla et al. [44] observed that smallamount of liquid oil formation started at temperature of 360–385 �C. The maximum liquid yield was collected at 469–494 �C.Onwudili et al. [34] observed that the oil conversion of LDPEstarted at 410 �C. A brown waxy material formed at temperaturebelow than 410 �C indicated the incomplete conversion of oil. Theyconcluded that the most optimum temperature to obtain the high-est liquid was at 425 �C for LDPE. In another study done by Marcillaet al. [26], they concluded that the most optimum temperature toobtain high liquid oil was at 550 �C. Further increase in tempera-ture to 600 �C only reduced the liquid yield obtained [45]. Hence,it can be summarized that the LDPE thermal degradation occurredat temperature range of 360–550 �C.

PP had lower thermal degradation temperature if compared toHDPE. According to Jung et al. [15] who studied the effect of tem-perature on HDPE and PP pyrolysis in a fluidized bed reactor, theyfound that the main decomposition of HDPE and PP happenedwithin the range of 400–500 �C based on derivative thermo-gravimetry analysis (DTG) curves. However, it was observed thatthe weight loss of PP fraction started to occur at lower temperaturebelow 400 �C in comparison to the HDPE fraction. Marcilla et al.[43] discovered that the maximum degradation temperature forPP was 447 �C while HDPE was 467 �C where the major weight losshappened. Theoretically, PP degraded faster than HDPE since halfof the carbon in PP chain is tertiary carbon, consequently easethe formation of tertiary carbocation during the degradation [15].

Among all plastics, PS degraded at the lowest temperature dur-ing pyrolysis process. Onwudili et al. [34] have investigated the PSpyrolysis in a batch reactor. From their studies, they found that noreaction seems to take place at 300 �C. However, they found that PSdegraded completely into highly viscous dark-colored oil at lowertemperature of 350 �C. The highest liquid oil was achieved at425 �C. The increase of temperature to 581 �C only reduced the liq-uid oil production and increased gaseous product [36]. Thus, it isworth noting that the thermal degradation temperature of PSwould be in the range of 350–500 �C approximately.

Therefore, it was proven that the temperature has the greatestimpact on reaction rate that may influence product compositionof liquid, gaseous and char for all plastics from the previous discus-sion. The operating temperature required relies strongly on theproduct preference. If gaseous or char product was preferred,higher temperature more than 500 �C was suggested. If liquidwas preferred instead, lower temperature in the range of 300–500 �C was recommended and this condition is applicable for allplastics.

3.2. Type of reactors

The type of reactors has an important impact in the mixing ofthe plastics and catalysts, residence time, heat transfer and effi-ciency of the reaction towards achieving the final desired product.Most plastic pyrolysis in the lab scale were performed in batch,semi-batch or continuous-flow reactors such as fluidized bed,fixed-bed reactor and conical spouted bed reactor (CSBR). Theadvantages and downsides of each reactor would be discussed inthe following subsections.

3.2.1. Batch and semi-batch reactorBatch reactor is basically a closed system with no inflow or out-

flow of reactants or products while the reaction is being carriedout. High conversion in batch reactor can be achieved by leavingthe reactant in the reactor for an extended time which is one ofits advantages. However, the disadvantages of batch reactor arethe variability of product from batch to batch, high labor costsper batch and the difficulty of large scale production [46].

In contrast, a semi-batch reactor allows reactant addition andproduct removal at the same time. The flexibility of adding reac-tants over time is an added advantage of the semi-batch reactorin terms of reaction selectivity. The disadvantage of semi-batchreactor is similar with the batch reactor in terms of labor cost, thusit is more suitable for small scale production.

Some researchers preferred to use batch reactors or semi-batchreactors in plastic pyrolysis laboratory scale experiment due to thesimplest design and ability to control the operating parameterseasily [47–56]. Pyrolysis in batch reactor or semi-batch reactornormally performed at temperature range of 300–800 �C for boththermal and catalytic pyrolysis. Some researchers added catalyststo the plastics to improve hydrocarbon yield and for productupgrading. In catalytic pyrolysis, the catalyst was mixed together


with the plastic sample inside the batch reactor. The drawback ofthis process would be a high tendency of coke formation on thesurface of the catalyst which reduced the catalyst efficiency overtime and caused high residue in the process. Besides that, it wasalso a challenge to separate the residue from the catalyst at theend of the experiment.

Sakata et al. [35] used batch reactor to study the pyrolysis of PPand HDPE at 380 �C and 430 �C accordingly using various catalystsand also without catalyst. The authors found that the liquid oilobtained from catalytic pyrolysis was even lower than the thermalpyrolysis for some catalysts. The liquid yield from PP in thermalpyrolysis was 80.1 wt% and from HDPE was 69.3 wt%. With theusage of several catalysts such as silica–alumina (SA-1) andHZSM-5, the liquid yield for both PP and HDPE reduced to 47–78 wt% and 49.8–67.8 wt% respectively. However, the usage of cer-tain catalysts such as silica–alumina (SA-2) and mesoporous silicacatalysts (FSM) improved the liquid yield for both plastics slightlythan the thermal pyrolysis with a very small increase of around1.0–7.0 wt%. Therefore, different catalysts might have differentreactivity to the plastic type. However, it has to be noted that thetendency of the coke formation on the catalyst surface also mightbe one of the reasons that degraded the effectiveness of the cata-lyst used in batch reactor over time.

Nevertheless, the direct contact of the catalyst with the plasticsin some cases may also improve the liquid yield. Abbas-Abadi et al.[57] conducted the PP pyrolysis in semi-batch reactor using FCCcatalyst at 450 �C. From the experiment, they found that very highliquid yield of 92.3 wt% was obtained. Some of the batch and semi-batch reactors were also equipped with stirrer that running at dif-ferent speed depends on the required setting as illustrated in Fig. 1.Seo et al. [58] studied the pyrolysis of HDPE using batch reactorequipped with stirrer at 450 �C. The stirrer speed was 200 RPM.Higher liquid oil was obtained than Sakata et al. [35] in thermalpyrolysis which was around 84.0 wt%. Besides that, the amountof liquid product obtained through catalytic pyrolysis using similarcatalyst of silica–alumina was also higher than Sakata et al. [35]which was 78 wt% while Sakata el al. [35] obtained 74.3 wt%.Therefore, it was clearly seen that the stirrer in the batch reactor

Fig. 1. Illustration of batch reactor

helped to enhance the liquid oil yield by improving the mixingfor the catalysts and plastics inside the reactor. More studies ofsemi-batch reactors with stirrer were also done by Abbas-Abadiet al. [59] and Kyong et al. [60,61].

Therefore, from the literature study, it was concluded that thebatch or semi-batch reactors are the best reactors to be used inthermal pyrolysis to obtain high liquid yield since the parameterscan be easily controlled. However, these reactors were not sug-gested for catalytic pyrolysis in consideration of the potential cokeformation on the catalyst outer surface that would disturb theoverall product yield. In addition, batch operation was not suitablefor large scale production since it required high operating cost forfeedstock recharging and thus, it was more appropriate for labora-tory experiment.

3.2.2. Fixed and fluidized bed reactorIn fixed-bed reactor, the catalyst is usually in palletized form

and packed in a static bed as shown in Fig. 2. It is easy to designbut there are some constraints such as the irregular particle sizeand shape of plastics as feedstock that would cause problem duringfeeding process. Besides, the available surface area of the catalystto be accessed by the reaction is also limited. However, there wereseveral researches chose to use fixed-bed reactor for the plasticpyrolysis [19,31,63–66]. In certain conditions, the fixed-bed reac-tors are merely used as the secondary pyrolysis reactor becausethe product from primary pyrolysis can be easily fed into thefixed-bed reactor which generally consists of liquid and gaseousphase [46]. Onu et al. [67] and Vasile et al. [68] used two-step pro-cess on the study of various plastic pyrolysis. However, there arevery few studies being done in two-step process as it is not costeffective and the results obtained are quite comparable with asingle-step process.

On the other hand, fluidized bed reactor solves some of theproblems occur in fixed-bed reactor. In contrast to fixed-bed reac-tor, the catalyst in fluidized bed reactor sits on a distributer plateas illustrated in Fig. 3 where the fluidizing gas passes through itand the particles are carried in a fluid state. Therefore, there isbetter access to the catalyst since the catalyst is well-mixed with

with stirrer equipment [62].

Fig. 2. Diagram of fixed-bed reactor [62].


the fluid and thus provides larger surface area for the reaction tooccur [69]. This reduces the variability of the process conditionswith good heat transfer. Besides, it is also more flexible than thebatch reactor since frequent feedstock charging can be avoidedand the process does not need to resume often. Hence, as for con-ventional design scale, fluidized bed reactor would be the bestreactor to be used in the pilot plant due to the lower operating cost.

Fig. 3. Diagram of fluidiz

Many researches preferred to use fluidized bed reactor in cat-alytic cracking of plastics over fixed bed reactor [27,37,45,70–76].Jung et al. [15] chose to use fluidized bed reactor for PP and PEpyrolysis because it provides almost a constant temperature withhigh mass and heat transfer, giving shorter residence time in thereactor and consequently more uniform spectrum of products.The plastic pyrolysis in fluidized bed reactors were carried out nor-mally at temperature as low as 290–850 �C for both thermal andcatalytic process. The comparison between HDPE and PP in cat-alytic degradation in fluidized bed reactor was studied by Luoet al. [77] using silica–alumina catalyst. The author reported thatthe liquid produced by PP was 87 wt% while HDPE produced lowerat 85 wt% liquid composition at 500 �C. This result was expectedsince HDPE had higher strength properties than PP.

Therefore, fluidized bed reactor is concluded to be the best reac-tor to perform catalytic plastic pyrolysis since the catalyst can bereused many times without the need of discharging, consideringcatalyst is a very expensive substance in the industry. Besides, itis more flexible than the batch reactor since frequent feedstockcharging can be avoided for continuous process and the processdoes not need to resume often. Hence, fluidized bed reactor wouldbe the most suitable reactor for large scale operation in terms ofeconomic point of view.

3.2.3. Conical spouted bed reactor (CSBR)Conical spouted bed reactor (CSBR) provides good mixing with

the ability to handle large particle size distribution, larger particlesand difference in particle densities [46]. There were some research-ers used CSBR for their catalytic cracking of plastic experiments[78–83]. Olazar et al. [81] claimed that CSBR had lower attritionand low bed segregation than the bubbling fluidized bed. It alsohad high heat transfer between phases and minor defluidizationproblem when handling sticky solids. However, a variety of techni-cal challenges during operation of this reactor have been encoun-tered such as catalyst feeding, catalyst entrainment and product(solid and liquid) collection that make it less favorable [46]. Addi-tionally, its complicated design that requires many pumps to be

ed bed reactor [62].

Fig. 4. Diagram of CSBR in HDPE pyrolysis using zeolite catalyst [84].


used in the system makes it unfavorable due to the high operatingcost involved.

Elordi et al. [84] used CSBR to conduct HDPE pyrolysis with HYzeolite catalyst at 500 �C that resulted in 68.7 wt% gasoline fraction(C5–C10) and the process was illustrated in Fig. 4. The gasoline hadan octane number of RON 96.5 which was closed with the standardgasoline quality. On the other hand, Arabiourrutia et al. [82]explored the waxes yield and characterization from HDPE, LDPEand PP pyrolysis at 450–600 �C using the CSBR. According to them,CSBR had the versatility of handling sticky solid that was hard tohandle in fluidized bed reactor. The spouted bed design was partic-ularly suitable for low temperature pyrolysis to obtain wax. Theauthors observed that the amount of waxes yield decreased withthe temperature. At higher temperature, more waxes are crackedinto liquid or gaseous product. HDPE and LDPE waxes productionwere very similar around 80 wt% while PP produced higher waxesat lower temperature about 92 wt%.

3.2.4. Microwave-assisted technologyThe recent interest in microwave technology offers a new tech-

nique for waste recovery via pyrolysis process. In this process, ahighly microwave-absorbent material such as particulate carbonis mixed with the waste materials. The microwave absorbentabsorbs microwave energy to create adequate thermal energy inorder to achieve the temperatures required for extensive pyrolysisto occur [85]. Microwave radiation offers several advantages overthe conventional pyrolysis method such as rapid heating, increasedproduction speed and lower production costs. Unlike conventionalmethods, microwave energy is supplied directly to the materialthrough the molecular interaction with the electromagnetic field,thus no time is wasted to heat up the surrounding area [86].Despite the advantages of microwave heating, there is also a majorlimitation which preventing this technology from being widelyexplored in industrial scale such as the absence of sufficient datato quantify the dielectric properties of the treated waste stream.The efficiency of microwave heating depends heavily on the dielec-tric properties of the material. For instance, plastics have low

dielectric constant and the mixture with carbon as the microwaveabsorber during pyrolysis may improve the energy absorbed to beconverted into heat in shorter time [85]. Therefore, the heatingefficiency may differ for each material and it has been a great chal-lenge to the industries.

The microwave heating was explored by Undri et al. [87] inpyrolysis process of polyolefin wastes (HDPE and PP) using twotypes of microwave absorbers which were carbon and tires. Differ-ent microwave power was used ranging from 1.2 to 6.0 kW. Fromthe results obtained, the highest liquid yield for HDPE was 83.9 wt% while PP was 74.7 wt%. It was observed that both experimentswere using carbon as the microwave absorber with microwavepower ranging from 3 to 6 kW. High power reduced the residencetime of the polymers in the oven, thus more polymers were con-verted into liquid rather than non-condensable gases. Using tiresas the microwave absorber increased the solid residue amountup to 33 wt%, in which accountable only to other substances in tirethat could not be pyrolyzed. In comparison with using carbon asthe microwave absorber, the solid residue accumulated was aslow as 0.4 wt% due to cocking process. Carbon material was a goodmicrowave absorbent that has high capacity to absorb and convertmicrowave energy into heat. Therefore, emphasis needs to be givenon the microwave power and absorber type in order to maximizethe liquid yield in microwave-induced pyrolysis process.

In other study, Ludlow-Palafox and Chase [88] have conducted amicrowave-induced pyrolysis on two different materials: HDPEpallets and toothpaste packaging which was in combination of alu-minum and polyethylene laminates. Carbon was used as the micro-wave absorber with 5 kW microwave power. This experiment wasquite different from others since a quartz vessel reactor with180 cm in diameter, equipped with 6 RPM impeller was placedinside the microwave. The product yield resulted from HDPE pyrol-ysis was recorded at temperature of 500–600 �C. Liquid oil col-lected was around 79–81 wt%, gaseous 19–21 wt% and 0 wt% ofsolid residue formed. For the aluminum and polyethylene lami-nates, no specific amount of product was reported. However, theauthors mentioned that there was no significant difference in the


product yield produced from the laminates and the HDPE pellets atthe same operating temperatures. Only the average molecularweight was slightly higher but the molecular weight distributionfor both cases were similar. In fact, the aluminum did not influencethe product yield since it was easily separated by sieving and beseen as a shiny clean surface. During the experiment, theyobserved a compound known as titanium dioxide which appearedas white powder adhered to the reactor side wall. Titanium oxideclearly presented in the painted surface of the toothpaste tube.This shows that this substance had no influence in the pyrolysisproduct since it was separated from the organic content of the lam-inate during pyrolysis. In conclusion, a real waste such as thetoothpaste packaging was successfully pyrolyzed throughmicrowave-induced pyrolysis method.

Several parameters that influenced the microwave heating per-formance in plastic pyrolysis such as the effect of nitrogen flowrate, different absorber type and microwave rotation design werealso explored by Khaghanikavkani [89]. The in-depth reviews ofmicrowave heating in pyrolysis have also been done by Fernandezet al. [86], Lam and Chase [85] and Undri et al. [90].

3.3. Pressure and residence time

The effect of pressure to the HDPE pyrolysis product was stud-ied by Murata et al. [91] in a continuous stirred tank reactor at ele-vated temperature of 0.1–0.8 MPa. Based on the studies, theydiscovered that the gaseous product increased tremendously fromaround 6 wt% to 13 wt% at 410 �C but only a small increase from4 wt% to 6 wt% at 440 �C as the pressure went up from 0.1 to0.8 MPa. This shows that pressure had high influence to the gas-eous product at higher temperature. Pressure also affected the car-bon number distribution of the liquid product by shifting to thelower molecular weight side when it was high. Besides, pressurealso had a significant effect on the rate of double bond formation.As reported by Murata et al. [91], the rate of double bond formationdecreased when pressure increased and this suggested that pres-sure directly affected the scission rate of C–C links in polymer.They also discovered that pressure had greater impact on residencetime at lower temperature. However, as the temperature increasedmore than 430 �C, the effect of pressure to the residence timebecame less apparent.

Residence time can be defined as average amount of time thatthe particle spends in the reactor and it may influence product dis-tribution [27]. Longer residence time increases the conversion ofprimary product, thus more thermal stable product is yielded suchas light molecular weight hydrocarbons and non-condensable gas[88]. Nevertheless, there is a temperature limitation in the processthat may influence the product distribution where until thatinstant, the residence time has not much effect on the product dis-tribution. Mastral et al. [92] studied the effect of residence timeand temperature on product distribution of HDPE thermal crackingin fluidized bed reactor. It was found that higher liquid yieldobtained at longer residence time (2.57 s) when the temperaturewas not more than 685 �C. However, the residence time had lessinfluence on the liquid and gaseous yield at higher temperatureabove 685 �C.

Therefore, it was concluded that pressure and residence timeare both temperature dependence factors that may have potentialinfluence on product distribution of the plastic pyrolysis at lowertemperature. Higher pressure increased the gaseous product yieldand affected the molecular weight distribution for both liquid andgaseous products but only apparent at high temperatures. Based onthe literature review, most researchers conducted their plasticpyrolysis studies at atmospheric pressure and focused more onthe temperature factor. The residence time was not brought upto attention while carrying out the experiment since the effect

would become less apparent at higher temperatures. Moreover,in terms of economic viewpoint, additional units such as compres-sor and pressure transmitter need to be added into the overall sys-tem, thus increase the operation cost if the factor of pressure isconsidered. However, it should be noted that these two factorsshould be put under consideration based on the product distribu-tion preference especially when running at temperature below450 �C.

3.4. Catalysts

3.4.1. Catalyst importance in pyrolysis of plasticsCatalyst speeds up chemical reaction but remains unchanged

towards the end of the process. Catalysts are widely used in indus-tries and researches to optimize product distribution and increasethe product selectivity. Hence, catalytic degradation is particularlyinteresting to obtain product of great commercial interest such asautomotive fuel (diesel and gasoline) and C2–C4 olefins, whichhave a huge demand in petrochemical industry [80]. When catalystis used, the activation energy of the process is lowered down, thusspeeds up the rate of reaction. Therefore, catalyst reduces the opti-mum temperature required and this is very crucial since the pyrol-ysis process requires high energy (highly endothermic) thathinders its commercial application. The usage of catalyst may helpin saving energy as heat is one of the most expensive costs inindustry. Besides that, catalyst was also used by many researchersfor product upgrading to improve the hydrocarbon distribution inorder to obtain pyrolysis liquid that had similar properties to theconventional fuel such as gasoline and diesel.

3.4.2. Type of catalystsThere are two types of catalyst which are homogeneous (only

one phase involve) and heterogeneous (involves more than onephase). Homogeneous catalyst used for polyolefin pyrolysis hasmostly been classical Lewis acid such as AlCl3 [93,94]. However,the most common type of catalyst used is heterogeneous sincethe fluid product mixture can be easily separated from the solidcatalyst. Hence, heterogeneous catalyst is economically preferablebecause various catalysts are quite costly and their reuse isdemanded. Heterogeneous catalyst can be classified as nanocrys-talline zeolites, conventional acid solid, mesostructured catalyst,metal supported on carbon and basic oxides [95]. Some examplesof nanocrystalline zeolites are HZSM-5, HUSY, Hb and HMORwhichare extensively used in the researches of plastic pyrolysis. Besides,the non-zeolites catalysts such as silica–alumina, MCM-41 and sil-icalite have also received much attention in current researches.Hence, the three types of catalysts that are widely used in plasticpyrolysis which are zeolites, FCC and silica–alumina catalystswould be discussed in the next subsections.

3.4.2.1. Zeolite catalyst. Zeolites are described as crystalline alumi-nosilicate sieves having open pores and ion exchange capabilities[96,97]. The structure is formed by three-dimensional frameworkwhere oxygen atoms link the tetrahedral sides. It is built by differ-ent ratio of SiO2/Al2O3 depends on its type. The ratio of SiO2/Al2O3

determines the zeolite reactivity which affects the final end pro-duct of pyrolysis.

Artetxe et al. [79] proven that the ratio of SiO2/Al2O3 of theHZSM-5 zeolite highly affected the product fraction yield in HDPEpyrolysis. Low ratio of SiO2/Al2O3 indicated the high acidity of thezeolite. The highest acidic catalyst (SiO2/Al2O3 = 30) was moreactive in cracking waxes, thus producing higher light olefins andlower heavy fraction of C12–C20 compared than the lowest acidiccatalyst (SiO2/Al2O3 = 280). The reduction of SiO2/Al2O3 ratio from280 to 30 improved the yield of light olefins from 35.5 to 58.0 wt% and decreased the yield of C12–C20 from 28.0 to 5.3 wt%.

Table 3Comparison of fuel properties of gasoline fraction obtained using three type of HZSM-5 with different ratio of SiO2/Al2O3 [79].

SiO2/Al2O3 Octane number Olefins (vol%) Aromatics (vol%) Benzene (vol%)

30 94.1 33.1 43.3 4.280 86.7 61.2 13.5 1.3280 85.9 68.9 6.9 0.46Required 95 <18 <35 <1


Besides, the reduction of SiO2/Al2O3 ratio in zeolite also raises theyield of light alkanes and aromatics. Table 3 compares the fuelproperties of gasoline fraction obtained with three type of HZSM-5 zeolite which having different ratio of SiO2/Al2O3. As depicted,the highest acidity catalyst with the lowest ratio of SiO2/Al2O3

led to a higher octane number with high content of aromaticsand benzene, but lower concentration of olefins. Even though theoctane number was lower and the olefins, aromatics and benzenestandard exceeded the specification established by EuropeanUnion (EU), the absence of sulfur in the gasoline composition madeit possible to be blended with refinery stream to achieve the stan-dard outlined by EU.

Besides HZSM-5, some more examples of zeolite catalyst areHUSY and HMOR which are widely used in plastic catalytic pyrol-ysis. Garfoth et al. [70] investigated the efficiency of different zeo-lite catalysts to the HDPE pyrolysis which were HZSM-5, HUSY andHMOR with polymer to catalyst ratio of 40 wt%. In their studies, itwas found that HZSM-5 had higher catalytic activity than HUSYand HMOR, referring to the very less residue left by HZSM-5around 4.53 wt% while HUSY and HMOR were leaving about7.07 wt% and 8.93 wt% residues behind respectively. This showsthat HZSM-5 able to maximize the total product conversion inplastic pyrolysis over other zeolites.

In terms of product selectivity, different zeolites may have dif-ferent product preferences. Marcilla et al. [98] studied the HZSM-5and HUSY performance on HDPE and LDPE at constant temperatureof 550 �C and 10 wt% polymer to catalyst ratio in a batch reactor.Higher liquid oil was recovered when using HUSY catalyst(HDPE = 41.0 wt%, LDPE = 61.6 wt%) compared with HZSM-5 cata-lyst (HDPE = 17.3 wt%, LDPE = 18.3 wt%) Oppositely, higher gas-eous product obtained when using HZSM-5 catalyst(HDPE = 72.6 wt%, LDPE = 70.7 wt%). This proves that different cat-alysts have different product selectivity. The same trend of productselectivity was also reported by Lin and Yen [71] on PP pyrolysisusing the HZSM-5 and HUSY zeolites.

Seo et al. [58] also investigated the effect of HZSM-5 in HDPEpyrolysis with catalyst to polymer ratio of 20 wt% at 450 �C. Theyobserved that HZSM-5 produced very less liquid yield around35 wt% but higher gaseous product of 63.5 wt%. Hernández et al.[99] obtained even lesser liquid yield around 4.4 wt% and 86.1 wt% gaseous product when running the HDPE pyrolysis at 500 �C withsame catalyst to polymer ratio as Seo et al. [58]. Lin and Yen [71]also obtained very small amount of liquid product in PP pyrolysisat 360 �C with catalyst to polymer ratio of 40 wt% using HZSM-5and HUSY zeolite catalysts. The liquid yield obtained was only2.31 wt% and 3.75 wt% respectively. Nevertheless, HZSM-5 hadhigher resistance to coking than HUSY catalyst when the productstream such as isobutane and isopentanes remained unaffectedthroughout the process while the olefins (butane and pentene)increased [71,100,101].

Besides that, the usage of zeolite catalyst in pyrolysis of realmunicipal plastic waste may also help to reduce the impurities inthe oil produced and this was proven in the study conducted byMiskolczi et al. [102]. In this study, the source of waste HDPEand PP were collected from agriculture and packaging sectors. Both

plastics were washed and shredded before pyrolysis. From theproperties analysis, both plastics contained sulfur(HDPE = 238 mg/kg, PP = 35 mg/kg). However, more impuritiessuch as nitrogen, phosphorus and calcium (963 mg/kg, 47 mg/kg,and 103 mg/kg) additionally found in the HDPE waste obtainedfrom the agriculture. The impurities were most probably comefrom the fertilizer which contained ammonium nitrate and super-phosphate which could be accumulated on the HDPE waste thatfailed to be removed by washing procedure. The catalytic pyrolysiswas done at 520 �C in the presence of 5 wt% HZSM-5 catalyst. Afterthe pyrolysis was done, the used catalyst structure was analyzedby SEM and EDAX. Results showed that trace of sulfur, nitrogenand phosphorus were attached on the catalyst surface besides ofthe silica–alumina structure of HZSM-5 zeolite (silica, aluminum,oxygen, magnesium, potassium, sodium and calcium). This indi-cates the elements of impurities came from the waste plastic. Nev-ertheless, the impurities attached on the catalyst surface did notaffect the product properties because the properties were ratheraffected by the grain diameter or catalyst pore structures. In fact,the catalyst obtained from the waste plastic pyrolysis could bere-used since the pore diameter was found to be similar with thefresh catalyst [102]. In term of liquid oil properties, the usage ofcatalyst helped to reduce the impurities content in the oil. Thiswas clearly shown in the results when the sulfur content in HDPEwaste pyrolysis reduced tremendously from 75 mg/kg to 37 mg/kgwith the usage of HZSM-5 catalyst. The same reduction trend wasalso observed for nitrogen and phosphorus content. No calciumcontent was found in the gasoline and light oil fraction, while itcould be only found in the heavy oil fraction.

Besides direct cracking of plastics, some researchers have alsoanalyzed the zeolite catalyst performance in two-step reactionprocess involving thermal and catalytic reactors sequentially[68,103]. Aguado et al. [33] have explored the catalytic conversionof LDPE in two-step reaction process consisted of batch and fixedbed reactor. The thermal cracking of plastic would take place inthe batch reactor and in the meanwhile, the vapors generated werecarried over to the fixed-bed reactor where the 10 wt% HZSM-5catalyst was placed. The pyrolysis was conducted within 425–475 �C. From the results, it was observed that the catalytic reform-ing over zeolite catalyst led to a significant increase of the gas frac-tion. The gaseous product had risen to 74.3 wt% at the highesttemperature whereas only 21.9 wt% of liquid hydrocarbons wascollected. Therefore, the trend of result observed was very similarto the direct catalytic cracking which produced high gaseous pro-duct over the usage of HZSM-5 catalyst.

As a final conclusion, it is worth noting that the usage of zeolitecatalyst in plastic pyrolysis only maximized the production of vola-tile hydrocarbon. As for higher efficiency and longer cycle timeusage, HZSM was recommended since the deactivation rate ofthe catalyst was extremely low and thus, more efficient forregeneration.

3.4.2.2. FCC catalyst (fluid catalytic cracking). FCC catalyst is made ofzeolite crystals and non-zeolite acid matrix known as silica–alu-mina with the binder [97,104–106]. The main component of FCCcatalyst for over 40 years is Zeolite-Y due to its high product selec-tivity and thermal stability [107]. FCC catalyst is normally used inthe petroleum refining industry to crack heavy oil fractions fromcrude petroleum into lighter and more desirable gasoline and liq-uid petroleum gas (LPG) fractions [107].

FCC catalyst that has been used is often known as ‘spent FCCcatalyst’ and usually can be obtained from commercial FCC processin petroleum refineries. It comes with different level of contamina-tion yet still valuable and can be reused in the pyrolysis process.Kyong et al. [60] investigated the effect of spent FCC catalyst onthe pyrolysis of HDPE, LDPE, PP and PS in stirred semi-batch reac-


tor at 400 �C. 20 g of catalyst was added into 200 g of reactants andheated up at rate of 7 �C/min. As a result, all plastics producedmore than 80 wt% liquid oil with PS being the highest (around90 wt% liquid yield). The liquid yields based on the plastic typeswere arranged in this order: PS > PP > PE (HDPE, LDPE). The gas-eous product yield had a reverse order with that of liquid in thisfollowing order: PE > PP > PS. This shows that PS was less crackedto the gaseous product since PS contained benzene ring that cre-ated more stable structure. Overall, it is concluded that spent FCCcatalyst still has high catalytic performance with the liquid yieldobtained above 80 wt% for all plastic samples. Additionally, it ismore cost effective since it is a ‘reused’ catalyst.

The effectiveness of spent FCC catalyst over thermal pyrolysis ofHDPE without catalyst was further explored by Kyong et al. [108]using exactly the same parameters as Kyong et al. [60] but thistime with higher temperature of 430 �C. From the experiment, itwas found that the liquid oil yield was increased slightly from75.5 to 79.7 wt% while the gaseous product reduced slightly from20.0 to 19.4 wt% when the catalyst was used. However, the amountof solid residue left reduced drastically from 4.5 to 0.9 wt% withpresence of the catalyst. Moreover, the catalytic pyrolysis also low-ers down the reaction temperature of HDPE when the initial liquidformation was observed at about 350 �C. In the non-catalyzedpyrolysis, the initial liquid only formed after 30 min at 430 �C. Thisimplies that the usage of spent FCC catalyst in thermal pyrolysisincreased the rate of reaction besides improving the overall pro-duct conversion.

Apart from that, the limitation of the polymer ratio to the cata-lyst also needs to be considered in order to maximize the pyrolysisproduct conversion. Different ratio of HDPE to FCC catalyst wasinvestigated by Abbas-Abadi et al. [57] from the range of 10 to60 wt% at constant temperature of 450 �C using the semi-batchstirred reactor. From the study, it was found that the best optimumratio for higher conversion to liquid yield was at 20 wt% catalyst/polymer ratio. The liquid product obtained was very high at91.2 wt% with gaseous and coke around 4.1 wt% and 4.7 wt%respectively. As the catalyst/polymer ratio was increased morethan 20 wt%, more coke and gas were produced, thus liquid pro-duction was minimized. This shows that there was a certain con-straint of the catalyst/polymer ratio to enhance the productconversion especially liquid oil yield and reduced coke formationon the catalyst.

Besides that, different condition of FCC may also influence theproduct distribution of plastic pyrolysis. For instance, steamingFCC catalyst would change the catalyst structure and composition.This was proven by Olazar et al. [81] who conducted a study onfresh, mild and severe steaming of FCC catalyst. The mild steamingwas carried out at 760 �C for 5 h and the severe steaming at 816 �Cfor 8 h. The result showed that the catalytic performance of FCChas improved by steaming. Severe steaming of FCC increased theproduction of diesel fraction (comprised of C10+ hydrocarbon)and reduced the gaseous fraction (comprised of C1–C4 hydrocar-bon). On the other hand, the fresh FCC catalyst produced less dieselfraction while very high gaseous product. The product yield wassummarized in Table 4.

The effectiveness of FCC catalyst to the different types of plas-tics was studied by Kyong et al. [60] at 400 �C with the catalyst/polymer ratio of 10 wt%. They observed that the liquid yield

Table 4Product distribution of FCC in fresh and steaming state [81].

Type of FCC catalyst C1–C4 (gaseous) (wt%) C5

Fresh FCC 52 35Mild steaming 25 38Severe steaming 5 20

produced were in the range of 80–90 wt% for HDPE, LDPE, PP andPS. This result shows that FCC able to maximize the liquid oil yieldin pyrolysis process for most plastic types. Abbas-Abadi et al. [57]also obtained very high liquid oil yield about 92.3 wt% in PP pyrol-ysis at 450 �C with 10 wt% catalyst/polymer ratio.

In conclusion, the usage of FCC catalyst in plastic pyrolysis wasencouraged to maximize the liquid oil production. Additionally, theusage of ‘spent FCC catalyst’ was an added advantage in terms ofeconomical view. However, it should be noted that catalyst/poly-mer ratio cannot be more than 20 wt% to avoid the dominationof coke and gaseous product.

3.4.2.3. Silica–alumina catalyst. Silica–alumina catalyst is an amor-phous acid catalyst that contains Bronsted acid sites with an ioniz-able hydrogen atom and Lewis acid site, an electron accepting sites.The acid concentration of silica alumina catalyst is determined bythe mole ratio of SiO2/Al2O3. Unlike zeolite, the acid strength of sil-ica–alumina is determined oppositely in which the high ratio ofSiO2/Al2O3 indicates the high strength of acidity. For instance,SA-1 (SiO2/Al2O3 = 4.99) has higher acidity than SA-2 (SiO2/Al2O3 = 0.27) and both of them are the commercial silica–aluminaavailable in the market [109].

Different strength of acidity in catalyst has great influence inthe final end product of plastic pyrolysis. Sakata et al. [109]explored the effect of catalysts acidity (SA-1, SA-2, ZSM-5) on theproduct distribution of HDPE pyrolysis. The experiment was per-formed at 430 �C in a semi-batch reactor where 1 g of catalystwas mixed with 10 g of HDPE. The acidity of the catalysts aretested using NH3 temperature programmed desorption (TPD) andwere in this following order: SA-1 > ZSM-5 > SA-2. As a result, itwas observed that SA-2 catalyst with lower acidity producedhigher amount of liquid oil (74.3 wt%), followed by SA-1 (67.8 wt%) and ZSM-5 (49.8 wt%). ZSM-5 possessed strong acid sites, thusproduced more gaseous products than the other two acid catalystsbut very less liquid yield. Uddin et al. [32] have also studied theeffect of SA-2 to HDPE and LDPE at the same temperature and cat-alyst/polymer ratio. The results that they obtained for liquid oilyield were not far with the one obtained by Sakata et al. [109].HDPE and LDPE pyrolysis each produced 77.4 wt% and 80.2 wt%respectively when SA-2 catalyst was used. HDPE structure wasstronger than LDPE due to its linear chain, thus the lower amountof liquid yield obtained was expected.

Besides HDPE and LDPE, Sakata et al. [35] have investigated theeffectiveness of silica–alumina catalyst on PP at 380 �C, but thistime with different catalyst contact mode: liquid phase and vaporphase. As for the liquid phase, the catalyst was mixed togetherwith the PP pallets and loaded into the batch reactor. Oppositelyfor vapor phase, the catalyst was placed on the stainless steel netsuspended 10 cm from bottom of the reactor. From their study,they found that the catalyst in liquid mode produced higher liquidproduct (68.8 wt%) since the wax residue decomposed into lighterhydrocarbon over silica–alumina catalyst. On the other hand,higher gaseous product (35 wt%) found from the catalyst in vaporphase since the hydrocarbon further decomposed into gaseousproduct over silica–alumina catalyst. Therefore, catalyst modewas also an important factor that needs an attention since it influ-enced the final product distribution in plastic pyrolysis.

–C9 (medium gasoline) (wt%) C10+ (diesel) (wt%)

154070


Besides catalyst contact mode, the reactivity of catalyst can alsobe optimized under certain range of temperatures. Luo et al. [77]performed the HDPE and PP pyrolysis using silica–alumina catalystat higher temperature of 500 �C in fluidized bed reactor. The liquidoil obtained for HDPE was about 85.0 wt% while PP was around90 wt% which was higher than the studies conducted by Sakataet al. [35] and Uddin et al. [32]. This shows that temperature alsoplays an important role in maximizing the catalyst performancein order to increase the liquid oil production in plastic pyrolysisprocess.

As a final conclusion, the best catalyst to optimize liquid oil pro-duction in plastic pyrolysis would be FCC catalyst. FCC catalyst ableto produce high liquid yield above 90 wt% for HDPE and PP pyrol-ysis while the highest product yield by silica–alumina for HDPEand PP was within the range of 85–87 wt% [57,59,77]. This showsthat these two catalysts were comparable in terms of the liquidoil production but FCC had better catalytic performance. Besidesthat, the ‘spent FCC catalyst’ can also be used instead of freshFCC that made it more economically attractive.

3.5. Type and rate of fluidizing gas

Fluidizing gas is an inert gas (also known as carrier gas) whichonly engaged in transportation of vaporized products without tak-ing part in the pyrolysis. There are many type of fluidizing gas thatcan be used for the plastic pyrolysis such as nitrogen, helium,argon, ethylene, propylene and hydrogen. Each type of fluidizinggas has different reactivity based on its molecular weight. Abbas-Abadi et al. [57] reported that the molecular size of the carriergas helped in determining the product composition and alsodependent on the temperature. Table 5 shows that the molecularweight of the carrier gas did affect the product distribution of cat-alytic PP pyrolysis. The lighter gas able to produced high amount ofcondensed product which was liquid oil. As depicted in Table 5, H2

produced the highest liquid yield of 96.7 wt% while without anycarrier gas, only 33.8 wt% liquid was yielded. This proves theimportance of carrier gas in enhancing the product yield in pyrol-ysis process. Besides that, it was also observed that the reactivity ofthe carrier gas influenced the coke formation. H2 coke yield wasvery minimal which was about 0.3 wt%, followed by ethylene,helium and propylene. Ethylene and nitrogen were having thesame molecular weight. However, ethylene seems to producehigher amount of liquid yield and lower coke formation than nitro-gen. This is because ethylene is more reactive than nitrogen that itcould shift the equilibrium to produce more liquid yield [57]. Nev-ertheless, of all those gases, nitrogen was commonly used by mostresearchers as fluidizing gas in plastic pyrolysis since it was easierand safer to handle than the high reactivity gas like hydrogen andpropylene due to their flammability hazard. Besides, even though

Table 5The effect of carrier gas on the product yield and the condensed product composition [57

Carrier gas Molecularweight

Condensedproduct yield (%)

Non-condensableproduct yield(%)

Coke yield(%)

H2 2 96.7 3 0.3He 4 94.7 3.2 2.1N2 28 92.3 4.1 3.6Ethylene 28 93.8 5.1 1.1Propylene 42 87.8 9.7 2.5Ar 37 84.8 9.8 5.4No carrier gas 51.3 33.8 14.9 n.d.a

T: 450 �C, stirrer rate: 50 r min�1, catalyst/PP = 0.1.a Not determined.

helium able to produce high liquid yield after hydrogen, it wasrarely used since the availability was limited and more expensivethan nitrogen.

Besides type of fluidizing gas, the fluidizing flow rate also mayinfluence the final product distribution. Lin and Yen [71] investi-gated the effect of different fluidizing gas rate on product distribu-tion of PP pyrolysis over HUSY catalyst at 360 �C. They found thatthe rate of degradation dropped instantly at the lowest fluidizingflow rate of 300 ml/min. The contact time for primary product ishigh at lower flow rate, causing the formation of coke precursor(BTX) to increase with the secondary product obtained eventhough the overall degradation rate is slower [110]. This was indi-cated by the high residue left when lower fludizing flow rate wasapplied. The gasoline and hydrocarbon gases fraction were alsomaximized at the highest fluidizing flow rate of 900 ml/min.Hence, the type and rate of fluidizing gas are also very importantin plastic pyrolysis as they clearly influence product distributionas discussed above.

4. Characteristics of plastic pyrolysis oil

4.1. Physical properties

Table 6 summarized the fuel properties of the liquid oil pro-duced in pyrolysis process. The experimental calorific value ofHDPE, PP and LDPE are all above 40 MJ/kg and were consideredhigh for energy utilization. According to Ahmad et al. [23], the cal-culated calorific value for both HDPE and PP were above 45 MJ/kg,and thus very closer to the commercial fuel grade criteria of gaso-line and diesel. The calorific value of PS was commonly lower thanthe polyolefin plastic due to the existence of the aromatic ring inthe chemical structure which had lesser combustion energy thanthe aliphatic hydrocarbon [34]. Overall, PET and PVC had the low-est calorific value below 30 MJ/kg due to the presence of benzoicacid in PET and chlorine compound in PVC that deteriorated thefuel quality. Benzoic acid also consisted of aromatic ring thatexplained the low calorific value in PET.

API gravity was the method used for measuring the density ofthe petroleum relative to water which was established by theAmerican Petroleum Institute (API) and was also known as specificgravity [116]. The API gravity of HDPE and PP were 27.48 and 33.03respectively while the densities were 0.89 and 0.86 g/cm3 accord-ingly [23]. The API gravity of PVC was very close to the diesel APIgravity value which was 38.98. On the other hand, the API gravityof LDPE was approaching the gasoline standard value which was47.75. Therefore, all of these values were comparable to the com-mercial diesel fuel except LDPE which was comparable to the stan-dard gasoline. In terms of density, all values seem comparable withthe commercial standard value of both gasoline and diesel.

].

Olefins(%)

Paraffins(%)

Naphthenes(%)

Aromatics(%)

Olefins/paraffinratio

30.86 46.53 20.54 2.07 0.6643.32 33.41 19.29 3.98 1.344.63 32.87 17.23 5.27 1.3641.76 34.76 19.75 3.73 1.242.36 31.85 20.92 4.87 1.3345.21 25.27 21.93 7.59 1.78n.d. n.d. n.d. n.d. n.d.

Table 6Fuel properties of plastic pyrolysis oil.

Physical properties Type of plastics (experimental typical value) Commercial standard value(ASTM 1979)

PET [19,111] HDPE [23] PVC [19,112] LDPE [113] PP [23] PS [114,115] Gasoline [23] Diesel [23]

Calorific value (MJ/kg) 28.2 40.5 21.1 39.5 40.8 43.0 42.5 43.0API gravity @ 60 �F n.a 27.48 38.98 47.75 33.03 n.a 55 38Viscosity (mm2/s) n.a 5.08a 6.36b 5.56c 4.09a 1.4d 1.17 1.9–4.1Density @ 15 �C (g/cm3) 0.90 0.89 0.84 0.78 0.86 0.85 0.780 0.807Ash (wt%) n.a 0.00 n.a 0.02 0.00 0.006 – 0.01Octane number MON (min) n.a 85.3 n.a n.a 87.6 n.a 81–85 –Octane number RON (min) n.a 95.3 n.a n.a 97.8 90–98 91–95 –Pour point (�C) n.a �5 n.a n.a �9 �67 – 6Flash point (�C) n.a 48 40 41 30 26.1 42 52Aniline point (�C) n.a 45 n.a n.a 40 n.a 71 77.5Diesel index n.a 31.05 n.a n.a 34.35 n.a – 40

n.a., not available in the literature.a Viscosity at 40 �C.b Viscosity at 30 �C.c Viscosity at 25 �C.d Viscosity at 50 �C.


The viscosity on the other hand was defined as a measurementof the fluid resistance to flow. Viscosity is very crucial in petroleumindustry since it determines how easy the oil can flow from thereservoir to the well during extraction process and also plays a cru-cial role in fuel injection process [23,116]. In Table 6, the viscosityvalues were determined at different temperatures as denoted atthe bottom of the table. Based on Table 6, it depicted the valueof kinematic viscosity of all plastics were very close with the vis-cosity of diesel except for PS which the viscosity value was closerto the gasoline viscosity. In terms of ash content, HDPE and PPhad negligible ash content and these indicated that the HDPEand PP pyrolysis oil was free from any metal contamination. Theash content in PS was also lower than the standard diesel whichwas less than 0.01 wt%. LDPE had slightly higher ash content of0.02 wt% but the value was still tolerable since the differencewas very minimal.

Besides that, the research octane number (RON) and motoroctane number (MON) which was important to characterize theanti-knock quality for the gasoline range (C6–C10) was also deter-mined. The high octane number indicates the better anti-knockquality that the fuel possesses. Knock is usually caused by the rapidcombustion of gasoline in an engine that produces an explosivenoise and degrades the engine performance over time [117]. There-fore, the anti-knock quality is very important to avoid engine dam-age. The MON and RON value for HDPE pyrolysis oil was 85.3 and95.3 respectively. PP pyrolysis oil had higher MON and RON valuewhich were 87.6 and 97.8 accordingly. The RON value for PS alsomatched the range of standard gasoline value which was in therange of 90–98. This suggests that the octane number of HDPE,PP and PS were comparable with commercial gasoline(MON = 81–85, RON = 91–95).

Pour point is known as the temperature at which the fluid stopsto flow [118]. Generally, the increase in viscosity may cause thefluid losses its flow characteristic. Liquid fuel that has lower pourpoint has lesser paraffin content but greater aromatic content[119]. HDPE, PP and PS pyrolysis oil had lower pour point around�5, �9 �C and �67 �C respectively than the commercial dieselwhich having the pour point of 6 �C. This indicates that the pyrol-ysis oil obtained from plastic pyrolysis were rich with aromaticcontent. This relates to the lower calorific value of HDPE, PP andPS pyrolysis oil in comparison with the commercial gasoline anddiesel.

One of the important properties in fuel handling to prevent firehazard during storage was the flash point. Flash point of the liquid

is defined as the lowest temperature at which the liquid mayvaporize and form a mixture in the air that ignites when an exter-nal flame is applied [120]. The flash point of HDPE, PVC and LDPEpyrolysis oil were very close to the commercial gasoline. This indi-cates that the flash point of those three plastics was comparable tothe light petroleum distillate fuel. The flash point of PP and PS werelower than both commercial gasoline and diesel. This shows thatPP and PS pyrolysis oil easier vaporized and thus need an extra pre-caution when handling.

Aniline point is a temperature at which the aniline compound(C6H5NH2) forms a single phase with the liquid oil [121]. Loweraniline point indicates the higher existence of aromatic compound.Oppositely, the higher aniline point indicates the higher amount ofparaffin compound in the oil. Olefin has the aniline point inbetween those aromatic and paraffin value [121]. Referring toTable 6, the aniline point of HDPE and PP pyrolysis oil were 45 �Cand 40 �C respectively which were much lower than the commer-cial gasoline and diesel. Gasoline and diesel were both having ani-line point of 71 �C and 77.5 �C. This once again proves that thepyrolysis oil obtained from HDPE and PP was rich with aromaticcompounds.

Diesel index evaluates the ignition quality of the diesel fuel inwhich the higher diesel index of the fuel indicates the higher qual-ity of the fuel [122–124]. The diesel index of the HDPE pyrolysis oilwas 31.05 while PP was 34.35. Even though the diesel index wasnot meeting the ASTM 1979 standard, the mixing of additives tofuel oil can improve the ignition quality of the diesel fuel andhas shown growing acceptance nowadays [125]. Therefore, Ahmadet al. [23] concluded that liquid product produced by HDPE and PPmet the commercial fuel grade and suggested to be a blend of gaso-line and diesel hydrocarbon range.

4.2. Chemical properties

Table 7 shows the main chemical compound from the plasticpyrolysis. The liquid oil composition was usually characterizedusing the FTIR and GC–MS equipment for detailed analysis. InPET pyrolysis, Cepeliogullar and Putun [19] reported that almosthalf of the liquid oil around 49.93% contained benzoic acid com-pound. The corrosiveness of the acid made it unfavorable to beused in thermochemical conversion system. As the reaction pro-ceeded during pyrolysis, PET tended to lose more aliphatic com-pound than the aromatic compound and this resulted in higherliquid yield than PVC. According to Cepeliogullar and Putun [19],

Table 7Main oil composition from the pyrolysis of plastics.

PET [19] HDPE [15] PVC [19] LDPE [45] PP[15] PS [34]

1-Propanone 1-Methylcyclopentene Azulene Benzene 2-Methyl-1-Pentene BenzeneBenzoic acid 3-Methylcyclopentene Naphthalene, 1-methyl- Toluene 3-Methylcyclopentene TolueneBiphenyl 1-Hexene Biphenyl Xylene 1-Heptene EthylbenzeneDiphenylmethane Cyclohexene Naphthalene, 1-ethyl- Dimethylbenzene 1-Octene Xylene4-Ethylbenzoic acid 1-Heptene Naphthalene, 1-(2-propenyl)- Trimethylbenzene C4–C13 hydrocarbon Styrene4-Vinylbenzoic acid 1-Octene Naphthalene, 2,7-dimethyl- Indane Over C14 hydrocarbon CumeneFluorene 1-Nonene Naphthalene, 1,6-dimethyl- Indene Benzene PropylbenzeneBenzophenone 1-Decene Naphthalene, 1,7-dimethyl- Methylindenes Toluene 2-Ethyltoluene4-Acetylbenzoic acid 1-Undecene Naphthalene, 1,4-dimethyl- Naphthalene Xylene NaphthaleneAnthracene 1-Tridecene Naphthalene, 1,6,7-trimethyl- Methylnaphthalenes Ethybenzene DiphenylmethaneBiphenyl-4-carboxylic acid C4–C13 hydrocarbon 9H-Fluorene Ethylnaphthalene Indene Anthracene1-Butanone Over C14 hydrocarbon Naphthalene, 1-(2-propenyl)- Dimethylnaphthalene Biphenyl 1,2-Diphenylethanem-Terphenyl Benzene Phenanthrene, 1-methyl Acenaphthylene 2,2-Diphenylpropane

Toluene Fluoranthene, 2-methyl- Acenaphthene 1,3-DiphenylpropaneXylene 1H-Indene, 2,3-dihydro-5-methyl- Trimethylnaphthalenes Phenylnaphthalene

Naphthalene, 2-phenyl- Fluorene DiphenylbenzeneTetramethylnaphthalene Triphenylbenzene


PET pyrolysis yielded 23.1 wt% liquid oil while PVC only produced12.3 wt% liquid oil. This shows that the liquid yield of PET almostdoubled the PVC. The low liquid yield in PVC indicated the highproduction of gaseous product during the pyrolysis. The structureof PVC that comprised of the halogen (Cl�) with high electronega-tivity explained the situation. High temperature during the pyrol-ysis would cause the dehydrochlorination process to occur,consequently increased the gaseous product and reduced the liq-uid oil yield [126]. The release of hydrochloric acid and chlorinecompound during the PVC pyrolysis indicated that the liquid oilwas not suitable to be used as fuel since it depreciated the fuelquality. Based on Table 7, it was clearly seen that most of thePVC compound was decomposed to naphthalene and its derivativearound 33.55% [19].

In PP and HDPE pyrolysis oil, Jung et al. [15] observed that theliquid oil contained primarily aliphatic, monoaromatic and pol-yaromatic compounds. As for the PP fraction, the increase in thetemperature reduced the aliphatic concentration in the oil to2.9 wt% at 746 �C. In contrast, high aliphatic concentration wasfound in HDPE pyrolysis oil around 20 wt% at 728 �C. This indicatesthe complexity of the HDPE structure to degrade during thermaldegradation process. Besides that, the BTX aromatics in PP pyroly-sis oil (53 wt%) were found higher than in the HDPE fraction (32 wt%) at the same temperature as mentioned previously. The mostabundant compound comprised in the BTX aromatics was the ben-zene. The concentration of benzene and toluene increased with thetemperature except xylene compound which did not have a signif-icant difference with the temperature. In terms of hydrocarbonproduct distribution, paraffins were the main product observed(66.55%) for PP derived liquid compared than HDPE (59.70%).Hence, PP pyrolytic oil was more value added than the HDPEderived liquid since paraffins released extra energy for combustionthan other hydrocarbon groups such as olefins and naphthenes.

As for the LDPE derived liquid oil, Williams and Williams [45]reported that the aliphatic compound which consisted of alkanes,alkenes and alkadienes was the main composition found. As thetemperature increased, the aliphatic concentration was in decreas-ing trend. However, the aromatic compound showed an oppositetrend in which the aromatic concentration increased with the tem-perature. Among the aromatic compound, benzene and tolueneconcentration showed a dramatic increase as the temperatureincreased except xylene and this observation matched the trendobserved in PP and HDPE pyrolysis as reported by Jung et al.[15]. However, it should be noted that the chemical concentrationdepends strongly on the pyrolysis operating temperature. Accord-ing to Williams and Williams [45], the oil contained no aromatic

and polyaromatic hydrocarbon at temperature of 500–550 �C. Nev-ertheless, a significant increase in the single ring aromatic com-pound and polycyclic aromatic compound (PAH) happened whenthe temperature increased to 700 �C that comprised around 25%of the liquid oil composition.

For PS pyrolysis oil, Onwudili et al. [34] reported that the ben-zene, toluene and ethyl benzene were three main components inthe PS oil product that increased with the temperature. On theother hand, styrene monomer kept decreasing with the tempera-ture and this suggest that the styrene radical formed during thedegradation process of PS was very reactive. Liu et al. [37] alsoreported the same observation. The styrene and monoaromaticswere among the major components in the liquid oil product thatthey covered around 80 wt% in the liquid fraction. These compo-nents were categorized in the low boiling point fraction of less orequal to 200 �C [37].

5. By-products of the plastic pyrolysis

Pyrolysis of plastics also produces char and gas as by-products.The proportion of by-product in pyrolysis strongly depends on sev-eral parameters such as temperature, heating rate, pressure andresidence time. Some information about the by-products gener-ated is discussed below.

5.1. Char

Generally, slow heating rate at very low temperature and longresidence time maximizes the char formation in pyrolysis process.Even though the char formation in fast pyrolysis process is com-monly low, it is worth noting the properties and usage of the charto fully maximize the potential of plastic pyrolysis. Jamradloedlukand Lertsatitthanakorn [127] analyzed the char propertiesobtained from the pyrolysis of HDPE plastic waste. From the prox-imate analysis, volatile matter and fixed carbon were found to bethe main components of the char (>97 wt%) while moisture andash were the minorities. These components were closely relatedto the proximate analysis of the raw plastic as tabulated in Table 1,showing that most plastics were composed from almost 99 wt% ofvolatile matter. The calorific value of the char was about 18.84 MJ/kg. Furthermore, the low sulfur content made it suitable to be usedas fuel, for instance in combustion with coal or other wastes.

Besides that, the char formation was found to be increased withthe temperature and this trend was observed by Jung et al. [15] inpyrolysis of PE and PP wastes. The char formation was increased


from 2 wt% to 4 wt% in PP pyrolysis and from 0.7 wt% to 2 wt% inPE pyrolysis as the temperature was raised from 668 �C to746 �C. Unfortunately, the char obtained from both plastics con-sisted mainly of inorganic matters up to 98.9 wt% which originatedfrom the inorganic substance in the feed fraction. In this case, thehigh inorganic matters caused the application of char as fuel to bedifficult. However, it still has potential to be used as road surfacingand as a building material [15].

Nevertheless, char can also be used as an adsorbent in watertreatment to remove heavy metal through an upgrading treatment.A multistep upgrading of chars was studied by Bernando [128] in aco-pyrolysis of PE, PP and PS plastic wastes, pine biomass and usedtires. The adsorption properties of the upgraded chars were exam-ined and the result indicated that the chars were mainly meso-porous and macroporous material with adsorption capacities formethylene blue dye in the range of 3.59–22.2 mg/g. This indicatesthat the upgraded chars should have good adsorption propertiestowards bulky molecules. Therefore, the upgrading treatments per-formed on the chars allowed carbonaceous residue from pyrolysisto be reused as precursors for adsorbents to be obtained. Otherpotential applications of pyrolysis char include using as feedstockin production of activated carbon and as solid fuel for boilers [86].

5.2. Gas

According to Prabir [129], high temperature and long residencetime were the best condition to maximize gas production in pyrol-ysis process. However, these conditions are opposite with theparameters to maximize oil production. Generally, gas productionin pyrolysis process of polyolefins and PS plastics were quite low inthe range of 5–20 wt% and it is strongly depends on the tempera-ture and type of plastics used in pyrolysis. The effect of tempera-ture and plastic types were further studied by Onwudili et al.[34] in a pyrolysis of LDPE, PS and their mixture. At 350 �C, itwas discovered that the gas product from the mixture was morethan the pyrolysis of individual plastic. The gas continued toincrease to 8.6 wt% at 425� where at this point, the gas productwas higher than pyrolysis of PS alone but lower than LDPE. Atthe same temperature, pyrolysis of PS produced some amount ofchar but not any significant gas product. However, the pyrolysisof LDPE did produce more gas but no char at this temperature.Hence, the authors noted that the amount of gas produced fromthe mixture was significantly contributed by the LDPE component,whereas char formation related closely to PS. At 450 �C, the gasproduction increased continuously to 12.8 wt% for the plasticmixture.

On the other hand, the pyrolysis of PET and PVC plastics pro-duced large amount of gases in comparison to other polyolefinplastics and PS. Cepeliogullar and Putun [19] discovered that theamount of gas produced in pyrolysis of PET (76.9 wt%) and PVC(87.7 wt%) waste plastics were much higher than the liquid yieldat 500 �C. The same trend was observed by Fakhrhoseini andDastanian [5] in the pyrolysis of PET at the same temperaturewhen 52.13 wt% of gas product collected from the process. Thisindicates that PET and PVC decomposed mainly to gas product atthis temperature. PET used very less energy content to convert intoother chemical structures and this mechanism caused moregas production in the pyrolysis process. Conversely, a dehydrochlo-rination step might occur during the pyrolysis of PVC that causeda high amount of gas to be released rather than the liquidproduct.

The gas composition depends on the composition of feedstockmaterial. Williams and Williams [130] studied the pyrolysis ofHDPE, LDPE, PP, PS, PET and PVC individually and they found thatthe main gas components produced during pyrolysis of each plasticwere hydrogen, methane, ethane, ethene, propane, propene,

butane and butene. However, PET produced additional gas compo-nents of carbon dioxide and carbon monoxide while hydrogenchloride was also produced by PVC. The gas produced in pyrolysisprocess also has significant calorific value. Jung et al. [15] reportedthat the gas produced from the pyrolysis of PE and PP alone hadhigh calorific value between 42 and 50 MJ/kg. Thus, the pyrolysisgas had high potential to be used as heating source in pyrolysisindustrial plant. Additionally, the ethene and propene can be usedas chemical feedstock for the production of polyolefins if separatedfrom other gas components. The pyrolysis gas can also be used ingas turbines to generate electricity and direct firing in boilers with-out the need for flue gas treatment [86].

6. Discussion on plastic pyrolysis scenarios

This review showed that many researches have been done tostudy the potential of plastic pyrolysis process in order to producevaluable products such as liquid oil and the results were convinc-ing. This technique offers several advantages such as enhancing thewaste management system, reducing the reliability to fossil fuels,increasing energy sources and also prevents the contamination tothe environment. The technique can be executed at differentparameters that resulted in different liquid oil yield and quality.Besides that, this technique offers great versatility and better eco-nomic feasibility in terms of the process handling and the variabil-ity of the product obtained.

As mentioned in the paragraph above, various parameters couldinfluence the liquid oil yield and the most critical factor was thetemperature. Different plastics may have different degradationtemperature depends on their chemical structures. Therefore, theeffective temperatures for the liquid optimization in pyrolysisalso varied for each plastic and it also strongly dependent on otherprocess parameters. Such parameters include the type of catalystused, the ratio of catalyst/polymer and also type of reactorsoperated.

Table 8 summarized the optimum temperature required tooptimize liquid oil yield in thermal and catalytic pyrolysis at differ-ent conditions. Other affected parameters include the type of reac-tors, pressure, heating rate and pyrolysis duration for each type ofplastics. All experiments carried out were using nitrogen gas as thefluidizing medium. Based on Table 8, PET and PVC are two plasticsthat produced very low yield of liquid oil in comparison with otherplastic types, which made these plastics infrequently explored byresearchers. It also should be noted that not all plastic types arerecommended for pyrolysis. PVC was not preferred in pyrolysissince it produced the major product of harmful hydrochloric acidand very low yield of liquid oil. Additionally, the pyrolysis oil alsocontained chlorinated compound that would degrade the oil qual-ity and also toxic to the environment.

As summarized in Table 8, it can be concluded that the mosteffective temperature to optimize the liquid oil yield in plasticpyrolysis would be in the range of 500–550 �C for thermal pyroly-sis. However, with the usage of catalyst in the pyrolysis, the opti-mum temperature could be lowered down to 450 �C and higherliquid yield was obtained. In most plastics, the usage of catalystin the process might improve the liquid oil yield, but PS was excep-tional. This is because PS degraded very easily without the needs ofany catalysts to speed up the reaction and yet 97 wt% of oil wasproduced [34]. Therefore, PS was the best plastic for pyrolysis sinceit produced the highest amount of liquid oil production among allthe plastics. As for the polyolefin plastic type, LDPE produced thehighest liquid oil yield (93.1 wt%), followed by HDPE (84.7 wt%)and PP (82.12 wt%) in thermal pyrolysis. However, with additionof catalyst such as FCC and at the right operating temperature,the liquid yield could be further maximized to above 90 wt%.

Table 8Summary of studies on plastic pyrolysis.

Reference* Type ofplastic

Reactor Process parameters Yield Others

Temperature (�C) Pressure Heating rate(�C/min)

Duration(min)

Oil(wt%)

Gas(wt%)

Solid(wt%)

[19] PET Fixed bed 500 – 10 – 23.1 76.9 0[5] PET – 500 1 atm 6 – 38.89 52.13 8.98[23] HDPE Horizontal steel 350 – 20 30 80.88 17.24 1.88[60] HDPE Semi-batch 400 1 atm 7 – 82 16 2 Stirring rate 200 RPM,

FCC catalyst 10 wt%[48] HDPE Batch 450 – – 60 74.5 5.8 19.7[59] HDPE Semi-batch 450 1 atm 25 – 91.2 4.1 4.7 Stirring rate 50 RPM,

FCC catalyst 20 wt%[77] HDPE Fluidized bed 500 – – 60 85 10 5 Silica alumina catalyst[26] HDPE Batch 550 – 5 – 84.7 16.3 0[27] HDPE Fluidized bed 650 – – 20–25 68.5 31.5 0[19] PVC Fixed bed 500 – 10 – 12.3 87.7 0[29] PVC Vacuum batch 520 2 kPa 10 – 12.79 0.34 28.13 Also yield HCl = 58.2 wt%[34] LDPE Pressurized batch 425 0.8–4.3 MPa 10 60 89.5 10 0.5[32] LDPE Batch 430 – 3 – 75.6 8.2 7.5 Also yield wax = 8.7 wt%[5] LDPE – 500 1 atm 6 – 80.41 19.43 0.16[31] LDPE Fixed bed 500 – 10 20 95 5 0[26] LDPE Batch 550 – 5 – 93.1 14.6 0[45] LDPE Fluidized bed 600 1 atm – – 51.0 24.2 0 Also yield wax = 24.8 wt%[23] PP Horizontal steel 300 – 20 30 69.82 28.84 1.34[35] PP Batch 380 1 atm 3 – 80.1 6.6 13.3[60] PP Semi-batch 400 1 atm 7 – 85 13 2 Stirring rate 200 RPM,

used FCC catalyst 10 wt%[57] PP Semi-batch 450 1 atm 25 – 92.3 4.1 3.6 Stirring rate 50 RPM,

used FCC catalyst 10 wt%[5] PP – 500 1 atm 6 – 82.12 17.76 0.12[36] PP Batch 740 – – – 48.8 49.6 1.6[60] PS Semi-batch 400 1 atm 7 – 90 6 4 Stirring rate 200 RPM, used FCC

catalyst, cat/poly = 10 w/w[34] PS Pressurized batch 425 0.31–1.6 MPa 10 60 97 2.50 0.5[49] PS Batch 500 – – 150 96.73 3.27 0 Used Zn catalyst, cat/poly = 5 w/w[36] PS Batch 581 – – – 89.5 9.9 0.6 64.9 wt% of liquid comprised of

styrene

* All experiments used nitrogen gas as fluidizing medium.


7. Conclusion

This review has provided concise summary of plastic pyrolysisfor each type and a discussion of the main affecting parametersto optimize liquid oil yield. Based on the studies on literatures,pyrolysis process was chosen by most researchers because of itspotential to convert the most energy from plastic waste to valuableliquid oil, gaseous and char. Therefore, it is the best alternative forplastic waste conversion and also economical in terms of opera-tion. The flexibility that it provides in terms of product preferencecould be achieved by adjusting the parameters accordingly. Thepyrolysis could be done in both thermal and catalytic process.However, the catalytic process provided lower operating tempera-ture with greater yield of liquid oil for most plastics with the rightcatalyst selection. The sustainability of the process is unquestion-able since the amount of plastic wastes available in every countryis reaching millions of tons. With the pyrolysis method, the wastemanagement becomes more efficient, less capacity of landfillneeded, less pollution and also cost effective. Moreover, with theexistence of pyrolysis method to decompose plastic into valuableenergy fuel, the dependence on fossil fuel as the non-renewableenergy can be reduced and this solves the rise in energy demand.

Acknowledgement

The authors would like to thank the University of Malaya forfully funding the work described in this publication through thePPP project number PG066-2015B. A special thanks is alsoextended to the Universiti Teknologi Mara (UiTM) and Ministryof Higher Education (MOHE) for providing financial support underthe Young Lecturer’s Scheme (TPM).

References

[1] Association of Plastic Manufacturers Europe. An analysis of European plasticsproduction, demand and waste data. Belgium: European Association ofPlastics Recycling and Recovery Organisations; 2015. p. 1–32.

[2] U.S. Environmental Protection Agency. Common wastes and materials. US;2014.

[3] Kukreja R. Advantages and disadvantages of recycling. Conserve EnergyFuture; 2009.

[4] Masanet E, Auer R, Tsuda D, Barillot T, Baynes A. An assessment andprioritization of ‘‘design for recycling” guidelines for plastic components. In:Electronics and the environment, IEEE international symposium; 2002. p. 5–10.

[5] Fakhrhoseini SM, Dastanian M. Predicting pyrolysis products of PE, PP, andPET using NRTL activity coefficient model. Hindawi Publishing Corporation;2013. p. 1–5.

[6] Bridgwater AV. Review of fast pyrolysis of biomass and product upgrading.Biomass Bioenergy 2012;38:68–94.

[7] Abnisa F, Wan Daud WMA. A review on co-pyrolysis of biomass: an optionaltechnique to obtain a high-grade pyrolysis oil. Energy Convers Manage2014;87:71–85.

[8] Kreith F. The CRC handbook of mechanical engineering. 2nd ed. CRC Press,Inc; 1998.

[9] Zannikos F, Kalligeros S, Anastopoulos G, Lois E. Converting biomass andwaste plastic to solid fuel briquettes. J Renew Energy 2013;2013:9.

[10] Heikkinen JM, Hordijk JC, de Jong W, Spliethoff H. Thermogravimetry as a toolto classify waste components to be used for energy generation. J Anal ApplPyrol 2004;71:883–900.

[11] Ahmad I, Ismail Khan M, Ishaq M, Khan H, Gul K, Ahmad W. Catalyticefficiency of some novel nanostructured heterogeneous solid catalysts inpyrolysis of HDPE. Polym Degrad Stab 2013;98:2512–9.

[12] Hong S-J, Oh SC, Lee H-P, Kim HT, Yoo K-O. A study on the pyrolysischaracteristics of poly(vinyl chloride). J Korean Inst Chem Eng1999;37:515–21.

[13] Park SS, Seo DK, Lee SH, Yu T-U, Hwang J. Study on pyrolysis characteristics ofrefuse plastic fuel using lab-scale tube furnace and thermogravimetricanalysis reactor. J Anal Appl Pyrol 2012;97:29–38.

[14] Aboulkas A, El harfi K, El Bouadili A. Thermal degradation behaviors ofpolyethylene and polypropylene. Part I: pyrolysis kinetics and mechanisms.Energy Convers Manage 2010;51:1363–9.

http://refhub.elsevier.com/S0196-8904(16)30061-9/h0015






























[15] Jung S-H, Cho M-H, Kang B-S, Kim J-S. Pyrolysis of a fraction of wastepolypropylene and polyethylene for the recovery of BTX aromatics using afluidized bed reactor. Fuel Process Technol 2010;91:277–84.

[16] Abnisa F, Daud WMAW, Sahu JN. Pyrolysis of mixtures of palm shell andpolystyrene: an optional method to produce a high-grade of pyrolysis oil.Environ Prog Sustain Energy 2014;33:1026–33.

[17] Othman N, Basri NEA, Yunus MNM, Sidek LM. Determination of physical andchemical characteristics of electronic plastic waste (Ep-Waste) resin usingproximate and ultimate analysis method. In: International conference onconstruction and building technology; 2008. p. 169–80.

[18] Çepeliogullar Ö, Pütün AE. Thermal and kinetic behaviors of biomass andplastic wastes in co-pyrolysis. Energy Convers Manage 2013;75:263–70.

[19] Cepeliogullar O, Putun AE. Utilization of two different types of plastic wastesfrom daily and industrial life. In: Ozdemir C, Sahinkaya S, Kalipci E, Oden MK,editors. ICOEST Cappadocia 2013. Turkey: ICOEST Cappadocia; 2013. p. 1–13.

[20] Wan Ho M. Waste plastics into fuel oil? UK: Institute of Science in Society;2015.

[21] Shioya M, Kawanishi T, Shiratori N, Wakao H, Sugiyama E, Ibe H, et al.Development of waste plastics liquefaction technology, feedstock recycling inJapan. In: Muller-Hagedorn M, Bockhorn H, editors. Feedstock recycling ofplastics, Germany; 2005. p. 19–42.

[22] Michael PA. Plastic waste total in MSW. Society of the Plastic Industry; 2010.[23] Ahmad I, Khan MI, Khan H, Ishaq M, Tariq R, Gul K, et al. Pyrolysis study of

polypropylene and polyethylene into premium oil products. Int J GreenEnergy 2014;12:663–71.

[24] Kumar S, Singh RK. Recovery of hydrocarbon liquid from waste high densitypolyethylene by thermal pyrolysis. Braz J Chem Eng 2011;28:659–67.

[25] Boundy B, Diegel SW, Wright L, Davis SC. Biomass energy data book. 4thed. US: Oak Ridge National Laboratory; 2011.

[26] Marcilla A, Beltrán MI, Navarro R. Thermal and catalytic pyrolysis ofpolyethylene over HZSM5 and HUSY zeolites in a batch reactor underdynamic conditions. Appl Catal B Environ 2009;86:78–86.

[27] Mastral FJ, Esperanza E, Garcia P, Juste M. Pyrolysis of high-densitypolyethylene in a fluidized bed reactor. Influence of the temperature andresidence time. J Anal Appl Pyrol 2001;63:1–15.

[28] British Plastics Federation. Polyvinyl chloride (PVC). London; 2015.[29] Miranda R, Jin Y, Roy C, Vasile C. Vacuum pyrolysis of PVC kinetic study.

Polym Degrad Stab 1998;64:127–44.[30] López A, de Marco I, Caballero BM, Laresgoiti MF, Adrados A. Dechlorination

of fuels in pyrolysis of PVC containing plastic wastes. Fuel Process Technol2011;92:253–60.

[31] Bagri R, Williams PT. Catalytic pyrolysis of polyethylene. J Anal Appl Pyrol2001;63:29–41.

[32] Uddin MA, Koizumi K, Murata K, Sakata Y. Thermal and catalytic degradationof structurally different types of polyethylene into fuel oil. Polym Degrad Stab1996;56:37–44.

[33] Aguado J, Serrano DP, San Miguel G, Castro MC, Madrid S. Feedstock recyclingof polyethylene in a two-step thermo-catalytic reaction system. J Anal ApplPyrol 2007;79:415–23.

[34] Onwudili JA, Insura N, Williams PT. Composition of products from thepyrolysis of polyethylene and polystyrene in a closed batch reactor: effects oftemperature and residence time. J Anal Appl Pyrol 2009;86:293–303.

[35] Sakata Y, Uddin MA, Muto A. Degradation of polyethylene and polypropyleneinto fuel oil by using solid acid and non-solid acid catalysts. J Anal Appl Pyrol1999;51:135–55.

[36] Demirbas A. Pyrolysis of municipal plastic wastes for recovery of gasoline-range hydrocarbons. J Anal Appl Pyrol 2004;72:97–102.

[37] Liu Y, Qian J, Wang J. Pyrolysis of polystyrene waste in a fluidized-bed reactorto obtain styrene monomer and gasoline fraction. Fuel Process Technol1999;63:45–55.

[38] Hopewell J, Dvorak R, Kosior E. Plastics recycling: challenges andopportunities. Philos Trans R Soc London B Biol Sci 2009:2115–26.

[39] Kaminsky W, Schlesselmann B, Simon CM. Thermal degradation of mixedplastic waste to aromatics and gas. Polym Degrad Stab 1996;53:189–97.

[40] Donaj PJ, Kaminsky W, Buzeto F, Yang W. Pyrolysis of polyolefins forincreasing the yield of monomers’ recovery. Waste Manage 2012;32:840–6.

[41] Sobko AA. Generalized Van der Waals-Berthelot equation of state. Dokl Phys2008;53:416–9.

[42] Chin BLF, Yusup S, Al Shoaibi A, Kannan P, Srinivasakannan C, Sulaiman SA.Kinetic studies of co-pyrolysis of rubber seed shell with high densitypolyethylene. Energy Convers Manage 2014;87:746–53.

[43] Marcilla A, García-Quesada JC, Sánchez S, Ruiz R. Study of the catalyticpyrolysis behaviour of polyethylene–polypropylene mixtures. J Anal ApplPyrol 2005;74:387–92.

[44] Marcilla A, Beltrán MI, Navarro R. Evolution of products during thedegradation of polyethylene in a batch reactor. J Anal Appl Pyrol2009;86:14–21.

[45] Williams PT, Williams EA. Fluidised bed pyrolysis of low density polyethyleneto produce petrochemical feedstock. J Anal Appl Pyrol 1998;51:107–26.

[46] Fogler HS. Elements of chemical reaction engineering. 4th ed. NewJersey: Pearson Education Inc.; 2010.

[47] Jan MR, Shah J, Gulab H. Catalytic degradation of waste high-densitypolyethylene into fuel products using BaCO3 as a catalyst. Fuel ProcessTechnol 2010;91:1428–37.

[48] Miskolczi N, Bartha L, Deák G, Jóver B, Kalló D. Thermal and thermo-catalyticdegradation of high-density polyethylene waste. J Anal Appl Pyrol2004;72:235–42.

[49] Adnan, Shah J, Jan MR. Thermo-catalytic pyrolysis of polystyrene in thepresence of zinc bulk catalysts. J Taiwan Inst Chem Eng 2014;45:2494–500.

[50] García RA, Serrano DP, Otero D. Catalytic cracking of HDPE over hybridzeolitic–mesoporous materials. J Anal Appl Pyrol 2005;74:379–86.

[51] Kim SS, Kim S. Pyrolysis characteristics of polystyrene and polypropylene in astirred batch reactor. Chem Eng J 2004;98:53–60.

[52] Cardona SC, Corma A. Tertiary recycling of polypropylene by catalyticcracking in a semibatch stirred reactor: use of spent equilibrium FCCcommercial catalyst. Appl Catal B Environ 2000;25:151–62.

[53] Adrados A, De Marco I, Caballero B, López A, Laresgoiti M, Torres A. Pyrolysisof plastic packaging waste: a comparison of plastic residuals from materialrecovery facilities with simulated plastic waste. Waste Manage2012;32:826–32.

[54] Uemura Y, Azeura M, Ohzuno Y, Hatate Y. Flash-pyrolyzed productdistribution of major plastics in a batch reactor. J Chem Eng Jpn2001;34:1293–9.

[55] Lee K-H, Shin D-H. Characteristics of liquid product from the pyrolysis ofwaste plastic mixture at low and high temperatures: influence of lapse timeof reaction. Waste Manage 2007;27:168–76.

[56] Shah J, Jan MR, Mabood F, Jabeen F. Catalytic pyrolysis of LDPE leads tovaluable resource recovery and reduction of waste problems. Energy ConversManage 2010;51:2791–801.

[57] Abbas-Abadi MS, Haghighi MN, Yeganeh H, McDonald AG. Evaluation ofpyrolysis process parameters on polypropylene degradation products. J AnalAppl Pyrol 2014;109:272–7.

[58] Seo Y-H, Lee K-H, Shin D-H. Investigation of catalytic degradation of high-density polyethylene by hydrocarbon group type analysis. J Anal Appl Pyrol2003;70:383–98.

[59] Abbas-Abadi MS, Haghighi MN, Yeganeh H. Evaluation of pyrolysis product ofvirgin high density polyethylene degradation using different processparameters in a stirred reactor. Fuel Process Technol 2013;109:90–5.

[60] Kyong HL, Nam SN, Dae HS, Seo Y. Comparison of plastic types for catalyticdegradation of waste plastics into liquid product with spent FCC catalyst.Polym Degrad Stab 2002;78:539–44.

[61] Lee KH. Composition of aromatic products in the catalytic degradation of themixture of waste polystyrene and high-density polyethylene using spent FCCcatalyst. Polym Degrad Stab 2008;93:1284–9.

[62] The University of York. Chemical reactors. The Essential Chemical Industry[online]. UK: The University of York; 2013.

[63] Saad JM, Nahil MA, Williams PT. Influence of process conditions on syngasproduction from the thermal processing of waste high density polyethylene. JAnal Appl Pyrol 2015;113:35–40.

[64] Renzini MS, Lerici LC, Sedran U, Pierella LB. Stability of ZSM-11 and BETAzeolites during the catalytic cracking of low-density polyethylene. J Anal ApplPyrol 2011;92:450–5.

[65] Choi SJ, Park Y-K, Jeong K-E, Kim T-W, Chae H-J, Park SH, et al. Catalyticdegradation of polyethylene over SBA-16. Korean J Chem Eng2010;27:1446–51.

[66] Ballice L. A kinetic approach to the temperature-programmed pyrolysis oflow-and high-density polyethylene in a fixed bed reactor: determination ofkinetic parameters for the evolution of n-paraffins and 1-olefins. Fuel2001;80:1923–35.

[67] Onu P, Vasile C, Ciocilteu S, Iojoiu E, Darie H. Thermal and catalyticdecomposition of polyethylene and polypropylene. J Anal Appl Pyrol1998;49:145–53.

[68] Vasile C, Pakdel H, Mihai B, Onu P, Darie H, Ciocalteu S. Thermal and catalyticdecomposition of mixed plastics. J Anal Appl Pyrol 2000;57:287–303.

[69] Kaminsky W, Kim J-S. Pyrolysis of mixed plastics into aromatics. J Anal ApplPyrol 1999;51:127–34.

[70] Garfoth AA, Lin YH, Sharratt PN, Dwyer J. Production of hydrocarbons bycatalytic degradation of high density polyethylene in a laboratory fluidized-bed reactor. Appl Catal A Gen 1998;169:331–42.

[71] Lin YH, Yen HY. Fluidised bed pyrolysis of polypropylene over crackingcatalysts for producing hydrocarbons. Polym Degrad Stab 2005;89:101–8.

[72] Marcilla A, Hernández MdR, García ÁN. Study of the polymer–catalyst contacteffectivity and the heating rate influence on the HDPE pyrolysis. J Anal ApplPyrol 2007;79:424–32.

[73] Lin YH, Yang MH, Yeh TF, Ger MD. Catalytic degradation of high densitypolyethylene over mesoporous and microporous catalysts in a fluidised-bedreactor. Polym Degrad Stab 2004;86:121–8.

[74] Mastral JF, Berrueco C, Gea M, Ceamanos J. Catalytic degradation of highdensity polyethylene over nanocrystalline HZSM-5 zeolite. Polym DegradStab 2006;91:3330–8.

[75] Sharratt P, Lin Y-H, Garforth A, Dwyer J. Investigation of the catalyticpyrolysis of high-density polyethylene over a HZSM-5 catalyst in a laboratoryfluidized-bed reactor. Ind Eng Chem Res 1997;36:5118–24.

[76] Yan R, Liang DT, Tsen L. Case studies—problem solving in fluidized bed wastefuel incineration. Energy Convers Manage 2005;46:1165–78.

[77] Luo G, Suto T, Yasu S, Kato K. Catalytic degradation of high densitypolyethylene and polypropylene into liquid fuel in a powder-particlefluidized bed. Polym Degrad Stab 2000;70:97–102.































































































































































[78] Elordi G, Olazar M, Castaño P, Artetxe M, Bilbao J. Polyethylene cracking on aspent FCC catalyst in a conical spouted bed. Ind Eng Chem Res2012;51:14008–17.

[79] Artetxe M, Lopez G, Amutio M, Elordi G, Bilbao J, Olazar M. Cracking of highdensity polyethylene pyrolysis waxes on HZSM-5 catalysts of differentacidity. Ind Eng Chem Res 2013;52:10637–45.

[80] Elordi G, Olazar M, Lopez G, Amutio M, Artetxe M, Aguado R, et al. Catalyticpyrolysis of HDPE in continuous mode over zeolite catalysts in a conicalspouted bed reactor. J Anal Appl Pyrol 2009;85:345–51.

[81] Olazar M, Lopez G, Amutio M, Elordi G, Aguado R, Bilbao J. Influence of FCCcatalyst steaming on HDPE pyrolysis product distribution. J Anal Appl Pyrol2009;85:359–65.

[82] Arabiourrutia M, Elordi G, Lopez G, Borsella E, Bilbao J, Olazar M.Characterization of the waxes obtained by the pyrolysis of polyolefinplastics in a conical spouted bed reactor. J Anal Appl Pyrol 2012;94:230–7.

[83] Aguado R, Olazar M, Gaisán B, Prieto R, Bilbao J. Kinetic study of polyolefinpyrolysis in a conical spouted bed reactor. Ind Eng Chem Res2002;41:4559–66.

[84] Elordi G, Olazar M, Aguado R, Lopez G, Arabiourrutia M, Bilbao J. Catalyticpyrolysis of high density polyethylene in a conical spouted bed reactor. J AnalAppl Pyrol 2007;79:450–5.

[85] Lam SS, Chase HA. A review on waste to energy processes using microwavepyrolysis. Energies 2012;5:4209–32.

[86] Fernandez Y, Arenillas A, Menandez JA. Microwave heating applied topyrolysis. Advances in induction and microwave heating of mineral andorganic materials. Spain: InTech; 2011.

[87] Undri A, Rosi L, Frediani M, Frediani P. Efficient disposal of waste polyolefinsthrough microwave assisted pyrolysis. Fuel 2014;116:662–71.

[88] Ludlow-Palafox C, Chase HA. Microwave-induced pyrolysis of plastic waste.Ind Eng Chem Res 2001;40:4749–56.

[89] Khaghanikavkani E. Microwave pyrolysis of plastic. J Chem Eng ProcessTechnol 2013;04.

[90] Undri A, Rosi L, Frediani M, Frediani P. Microwave pyrolysis of polymericmaterials. In: Chandra Usha, editor. Microwave heating. InTech; 2011.

[91] Murata K, Sato K, Sakata Y. Effect of pressure on thermal degradation ofpolyethylene. J Anal Appl Pyrol 2004;71:569–89.

[92] Mastral FJ, Esperanza E, Berrueco C, Juste M, Ceamanos J. Fluidized bedthermal degradation products of HDPE in an inert atmosphere and in air–nitrogen mixtures. J Anal Appl Pyrol 2003;70:1–17.

[93] Ivanova SR. Selective catalytic degradation of polyolefins. Prog Polym Sci1990;15:193–215.

[94] Stelmachowski M. Thermal conversion of waste polyolefins to the mixture ofhydrocarbons in the reactor with molten metal bed. Energy Convers Manage2010;51:2016–24.

[95] Aguado J, Serrano DP, Escola JM. Catalytic upgrading of plastic wastes infeedstock recycling and pyrolysis of plastic wastes, Spain; 2006.

[96] International Zeolite Association; 2005.[97] Degnan Jr TF. Applications of zeolites in petroleum refining. Top Catal

2000;13:349–56.[98] Marcilla A, Beltran MI, Navarro I. Thermal and catalytic pyrolysis of

polyethylene over HZSM-5 and HUSY zeolites in a batch reactor underdynamic conditions. Appl Catal B Environ 2008;86:78–86.

[99] Hernández MdR, Gómez A, García ÁN, Agulló J, Marcilla A. Effect of thetemperature in the nature and extension of the primary and secondaryreactions in the thermal and HZSM-5 catalytic pyrolysis of HDPE. Appl Catal AGen 2007;317:183–94.

[100] Uemichi Y, Hattori M, Itoh T, Nakamura J, Sugioka M. Deactivation behaviorsof zeolite and silica–alumina catalysts in the degradation of polyethylene. IndEng Chem Res 1998;37:867–72.

[101] Obeid F, Zeaiter J, Al-Muhtaseb AaH, Bouhadir K. Thermo-catalytic pyrolysisof waste polyethylene bottles in a packed bed reactor with different bedmaterials and catalysts. Energy Convers Manage 2014;85:1–6.

[102] Miskolczi N, Angyal A, Bartha L, Valkai I. Fuels by pyrolysis of waste plasticsfrom agricultural and packaging sectors in a pilot scale reactor. Fuel ProcessTechnol 2009;90:1032–40.

[103] Syamsiro M, Saptoadi H, Norsujianto T, Noviasri P, Cheng S, Alimuddin Z,et al. Fuel oil production from municipal plastic wastes in sequentialpyrolysis and catalytic reforming reactors. Energy Proc 2014;47:180–8.

[104] Magee JS, Mitchell MM. Fluid catalytic cracking: science andtechnology. Elsevier; 1993.

[105] Humphries A, Wilcox J. Zeolite components and matrix compositiondetermine FCC catalyst performance. Oil Gas J 1989;87.

[106] Rajagopalan K, Habib Jr E. Understand FCC matrix technology. HydrocarbProcess 1992;71.

[107] Marcilly CR. Where and how shape selectivity of molecular sieves operates inrefining and petrochemistry catalytic processes. Top Catal 2000;13:357–66.

[108] Kyong HL, Sang GJ, Kwang HK, Nam SN, Dae HS, Park J, et al. Thermal andcatalytic degradation of waste high density polyethylene (HDPE) using spentFCC catalyst. Korean J Chem Eng 2003;20:693–7.

[109] Sakata Y, Uddin MA, Muto A, Kanada Y, Koizumi K, Murata K. Catalyticdegradation of polyethylene into fuel oil over mesoporous silica (KFS-16)catalyst. J Anal Appl Pyrol 1997;43:15–25.

[110] Lin YH, Yang MH. Catalytic pyrolysis of polyolefin waste into valuablehydrocarbons over reused catalyst from refinery FCC units. Appl Catal A Gen2007;328:132–9.

[111] Sarker M, Kabir A, Rashid MM, Molla M, Din Mohammad ASM. Wastepolyethylene terephthalate (PETE-1) conversion into liquid fuel. J FundamRenew Energy Appl 2011;1:1–5.

[112] Manickaraja E, Tamilkolundu S. Catalytic degradation of waste PVC intoliquid fuel using BaCO3 as catalyst and its blending properties with dieselfuel. Discover 2014;23:74–8.

[113] Desai SB, Galage CK. Production and analysis of pyrolysis oil from wasteplastic in Kolhapur city. Int J Eng Res Gen Sci 2015;3:590–5.

[114] Pinto F, Costa P, Gulyurtlu I, Cabrita I. Pyrolysis of plastic wastes. 1. Effect ofplastic waste composition on product yield. J Anal Appl Pyrol 1998;51:39–55.

[115] Blazso M. Composition of liquid fuels derived from the pyrolysis of plastics.In: Scheirs J, Kaminsky W, editors. Feedstock recycling and pyrolysis of wasteplastics: converting waste plastics into diesel and other fuels. Hungary: JohnWiley & Sons; 2006. p. 315–42.

[116] Meyer RF, Attanasi ED. Heavy oil and natural bitumen-strategic petroleumresources. US: U.S. Geological Survey; 2003. p. 1–6.

[117] Kalghatgi GT. Fuel anti-knock quality – Part 1. Engine studies. Chester,UK: Shell Global Solutions; 2001.

[118] Bozbas K. Biodiesel as an alternative motor fuel: production and policies inthe European Union. Renew Sustain Energy Rev 2008;12:542–52.

[119] Schlumberger. Pour point. Oilfield glossary; 2015.[120] Horng JL, Chia LT, Jim SL. A model for predicting the flash point of ternary

flammable solutions of liquid. Combust Flame 2004;138:308–19.[121] Schlumberger. Aniline point test. Oilfield glossary; 2015.[122] Cookson DJ, Latten JL, Shaw IM, Smith BE. Property-composition relationships

for diesel and kerosene fuels. Fuel 1984;64:509–19.[123] Becker A, Fischer H. A suggested index of diesel fuel performance. SAE

technical paper; 1934.[124] Kumar S, Prakash R, Murugan S, Singh RK. Performance and emission analysis

of blends of waste plastic oil obtained by catalytic pyrolysis of waste HDPEwith diesel in a CI engine. Energy Convers Manage 2013;74:323–31.

[125] Nino EF, Nino TG. Fuels and combustion. Manila: Rex Bookstore, Inc.; 1997.[126] Kim S. Pyrolysis of PVC waste pipe. Waste Manage 2001;21:609–16.[127] Jamradloedluk J, Lertsatitthanakorn C. Characterization and utilization of

char derived from fast pyrolysis of plastic wastes. Proc Eng2014;69:1437–42.

[128] Bernando M. Physico-chemical characterization of chars produced in the co-pyrolysis of wastes and possible routes of valorisation. Portugal: ChemicalEngineering. Universidade Nova de Lisboa; 2011. p. 27–36.

[129] Prabir B. Biomass gasification and pyrolysis. Practical design andtheory. USA: Elsevier Inc.; 2010.

[130] Williams PT, Williams EA. Interaction of plastics in mixed-plastics pyrolysis.Energy Fuel 1998;13:188–96.

























































































































Energy Conversion and Management - UMEXPERT 178 · PDF fileReview A review on pyrolysis of...

Documents

Transcript of Energy Conversion and Management - UMEXPERT 178 · PDF fileReview A review on pyrolysis of...